Patent Publication Number: US-7212802-B2

Title: Method and system for providing a portable communication service

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is a continuation of U.S. patent application Ser. No. 09/302,159, filed Apr. 29, 1999, now U.S. Pat. No. 6,577,857, which is a continuation of U.S. patent application Ser. No. 08/369,003, filed Jan. 5, 1995, now U.S. Pat. No. 5,983,091 issued on Nov. 9, 1999, which are both herein incorporated by reference. 

   FIELD OF THE INVENTION 
   The present invention relates generally to the field of portable communication units, and more particularly to an improved portable communication unit that operates based upon discrete allocable blocks of airtime. 
   BACKGROUND OF THE INVENTION 
   With the advent of the cellular telephone communications systems, the portable communications industry has experienced tremendous growth. It is estimated that there are presently over 20 million Americans who own and operate a cellular telephone. In addition, there are several million Americans who have applied for service with a cellular telephone carrier, but have been denied service because of insufficient or a bad credit history. As a result, a significant segment of the potential market for the cellular telephone industry is excluded from receiving or even being considered for cellular telephone service. 
   The prior art discloses various techniques for operating a communication system base site to accept or deny a request for access from a communication unit. For example, U.S. Pat. No. 5,274,368 to Breeden et al., assigned to Motorola, Inc., discloses such a system where a communication base site processes predetermined identification codes to determine whether a particular request for access to the base site is authorized. U.S. Pat. No. 4,908,848 to Hanawa discloses another technique for operating the mobile communication system to restrict certain types of call operations. Again, the system proposed by Hanawa, like the Breeden et al. system, requires modification of the communication system base site to recognize particular unique codes associated with a request for service from a particular portable communication unit. The modifications suggested by such prior art systems, however, have not been widely nor consistently implemented. 
   Theft of portable communication service, particularly with respect to cellular telephone service, has been increasing at an alarming rate and now represents a major problem in the industry. In some cases, the portable communication service user is placed in an awkward position where hundreds or even thousands of dollars of unauthorized communication service has been charged to the account of the user. If the user is able to establish that such service was unauthorized, the portable communication service provider will lose the revenue for the service provided that was unauthorized. In addition to theft of service, theft of portable communication equipment has also been increasing. Portable communication equipment, which may not be recognized as stolen until significant unauthorized service has been used, is very costly to the user and the service provider. 
   SUMMARY OF THE INVENTION 
   The present invention addresses and overcomes the limitations of such prior art systems, by providing a unique portable communication unit that may be easily configured, for example by a dealer for such units, to provide portable communication service in discrete predetermined blocks of service time. A significant advance of the present invention over systems proposed by the prior art, is that the unique operational characteristics of the invention are implemented in the portable communication unit itself rather than the communication system base site. Thus, the unique features of the portable communication unit of the present invention operate independent of any communication system base site that it might communicate with. As a result, a portable communication unit in accordance with the present invention may, depending upon the configuration specified by the dealer, operate with all the functionality and versatility of a conventional portable communication unit, communicating with any number of different communication system base sites, depending upon the location of the portable communication unit. 
   Due to the limited amount of airtime available for the portable communication unit of the present invention at any particular time, the unit of the invention will not be desirable as a target for theft. For it is the substantial and virtually unlimited additional airtime that is the objective when a portable communication unit is stolen or used without authorization. 
   In addition to the foregoing advantages, the unique features of the present invention facilitate the creation of a new class of portable communication units, useful for a variety of unique and desirable service situations. For example, the portable communication unit of the present invention, which may operate for a predetermined yet limited period of time, is particularly suited to certain industries, such as for equipment repair or service personnel who are given a portable communication unit to facilitate communication during service calls. The limited amount of airtime is a feature that ensures that the unit is not used for significant periods of time when unauthorized. In addition, the portable communication unit of the present invention is also particularly well-suited for individuals with short term communications needs, such as on a rental basis, perhaps also in association with the rental of an automobile. 
   In accordance with one embodiment of the invention, when a user exhausts a block of airtime, the user may contact the dealer or manufacturer of the portable communication unit to obtain a code that may be entered to activate an additional block of airtime. Such codes are known to, or may be generated by the dealer, based upon several variables, some of which are unique to the particular portable communication unit. A dealer will, typically, require either a pre-payment or other assurance of payment before providing the user with a code to activate an additional block of airtime, particularly when the unit is used in the context of users who have special credit needs. When the correct code is input, an additional predetermined and discrete block of airtime is activated and may be used to operate the portable communication unit. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other objects of the present invention will become apparent from the following detailed description taken in conjunction with the drawings in which: 
       FIG. 1  is a graphical representation of a preferred environment for using the portable communication unit of the present invention; 
       FIG. 2  is a schematic flow diagram illustrating some of the unique operational features of the present invention; 
       FIG. 3  is a schematic flow diagram illustrating the Add Credit Routine utilized by the user to allocate additional airtime for the portable communication unit of the invention; 
       FIGS. 4A and 4B  are block diagrams showing the contents and organization of some of the information stored in the memory of the portable communication unit of the invention; and 
       FIG. 5  is a schematic flow diagram illustrating the Code Generation Routine used to generate a set of unique codes which are checked against codes entered by a user to allocate additional airtime. 
   

   While the invention will be described in connection with certain preferred embodiments, it should be understood that it is not intended to limit the invention to those particular embodiments. To the contrary, it is intended to cover all alternatives, modifications and equivalents falling within the spirit and scope of the invention as defined by the appended claims. In particular, in some areas the following detailed description is cast in terms of one embodiment of the invention in the form of an improved cellular telephone handset. Such a specific description is provided only by way of example, however, and the unique and advantageous features of the invention, which are defined by the claims, are applicable to a much wider class of portable communication units that transmit and receive audio, visual and video information, as well as any of a variety of digitally encoded data, through any number of conventional communication protocols. 
   DETAILED DESCRIPTION OF THE INVENTION 
   Referring first to  FIG. 1 , there is illustrated a graphical representation of one general environment for use of the unique portable communication unit  10  of the present invention. A portable communication unit user  12  may obtain a portable communication unit  10 , for example, from a commercial dealer  14  or similar vendor for portable communication equipment and services. Alternatively, the portable communication user  12  may separately purchase a portable communication unit  10  of the present invention and enter into an arrangement for service with a portable communication service dealer  14 . As will be apparent, the portable communication unit  10  of the invention is particularly well-suited for situations where the individual desiring portable communication service has an insufficient or poor credit history, effectively barring the individual from obtaining portable communication service offered by conventional means directly through any number of portable communication carriers or service providers. 
   According to one embodiment of the invention, the portable communication service user  12  will contact a dealer  14  that offers the sale or use of a portable communication unit  10  in accordance with the present invention. As will be described in much greater detail in the following, the portable communication unit of the invention provides portable communication service in discrete blocks of service time. Thus, upon initial purchase or rental of a portable communication unit  10  according to the invention, or upon initial activation of portable communication service, the user will be able to use the portable communication unit for a predetermined, yet limited amount of airtime. For example, the unit  10  will typically have one block of airtime available so that the user may be able to use the unit for an aggregate of 45 minutes of airtime. After the current block of service time has been used, according to one arrangement, the user  12  may contact the dealer  14  to obtain a unique authorization code to be entered into the unit  10  to enable another block of service airtime. Typically, communication with the dealer  14  to obtain a code for additional airtime will also involve the user  12  authorizing the dealer to charge the additional block of airtime to the user&#39;s account with the dealer  14 , or perhaps in some cases authorizing a charge to the credit card for the user  12 . In other cases, the dealer  14 , particularly with users that do not have an established or clean credit record, may require the user  12  to pre-pay for service time before the dealer  14  will provide the user  12  with codes for additional airtime. 
   According to one embodiment of the invention the portable communication unit  10  will operate based upon two different types of airtime. Home time airtime is used to operate the unit  10  when it is within the vicinity of a local service provider  16 . Roam time airtime, on the other hand, is used to operate the unit  10  when it is outside of the vicinity of a local or primary service provider  16 . Thus, the dealer  14  could be contacted by the user  12  to provide a code that could be used to activate an additional block of roam time airtime, or to provide a code that could be used to activate an additional block of home time airtime. As should be evident, in some cases certain users  12  may want to limit usage of their portable communication unit  10  for communication only within the vicinity of the local or primary service provider  16 , and will therefore only activate blocks of home time airtime to operate the portable communication unit  10 . 
   The portable communication unit  10  of the invention operates in a relatively conventional manner with respect to the portable communications service provider  16 , such as a service provider for cellular communications service. Thus, the unique and advantageous features of the portable communication unit  10  of the invention are essentially “transparent” to the portable communication carrier or service provider  16 . The particular portable communications carrier account or service contract  18  that provides service for the portable communication unit  10  of the invention, however, is, according to the illustrated arrangement, established and maintained by the dealer  14 , rather than the user  12 . As a result, a user  14  that otherwise might not meet the credit standards for an account with the service provider  16  may obtain portable communication service through the dealer  14  utilizing the portable communication unit  10  of the present invention that allocates airtime in discrete blocks. In some cases, of course, the service provider  16  could essentially “replace” the dealer  14  by establishing a similar relationship directly with the user  12  in conjunction with the unique portable communication unit  10  of the invention. 
   As can be seen in  FIG. 1 , the portable communication unit  10  according to the invention resembles and is nearly identical in appearance to a conventional cellular telephone. Like a conventional cellular telephone, the portable communication unit  10  includes a set of keys  26  that may be individually pressed, for example, to dial a number or to operate the unit  10  to perform other functions. As should be evident, the portable communication unit  10  depicted in  FIG. 1  is illustrated as a cellular telephone by way of example, and the unique and advantageous functional features of the present invention may be readily implemented with respect to other portable communication units, including, for example, digital personal communications systems (PCS) which have voice, data and image communication capability. Preferably, the portable communication unit  10  will include an alphanumeric display  20  for prompting the user  12  and providing the user  12  with information regarding the status and other information regarding the unit  10 . In addition, the portable communication unit  10  will preferably also include a menu key  22  which a user  12  can press to enter a menu system or otherwise specify a particular function or routine for the unit  10  to perform. For example, the menu key  22  would likely be pressed in combination with another key or keys to display to the user the remaining amount of home time airtime available in the unit. Another key combination with the menu key  22  could display the remaining amount of roam time airtime available in the unit. Similarly, the menu key  22  could be utilized with other key combinations to perform a routine where the user could enter a code for allocating an additional block of home time or roam time airtime. 
   According to a preferred embodiment of the invention, the portable communication unit will also be provided with at least one “emergency” or pre-authorized number which it can communicate with at any time, even if no additional home time or roam time airtime is available. Preferably, as illustrated, this emergency number will be preprogrammed by either the manufacturer or the dealer and can be immediately activated by pressing a dedicated “hot” key  24  provided on the unit  10 . According to an alternative embodiment of the invention, such a “hot” key  24  might also allow the user  12  to use the portable communication unit  10  to contact the dealer  14  in order to request a code to allocate an additional block of airtime. 
   Turning now to  FIG. 2 , there is illustrated in flow diagram format the general operation of the portable communication unit according to one embodiment of the invention. Operation commences in Step  50  when initial power is supplied to the portable communication unit. Next, in Step  52 , a general system initialization routine is performed where, perhaps, RAM memory for executing various portions of the routine is allocated and loaded with appropriate data and instructions from the PROM memory, and conventional diagnostic checks may be performed to ensure proper operation and configuration of the unit. In addition, certain general parameters relating to the unit itself are read from PROM memory, including, for example, the “Home” carrier station identification number in order for the unit to determine if it should operate in “Home” mode or in “Roam” mode. After initialization, the routine continues to Step  54  where, according to a conventional procedure, the portable communication unit operates to determine and select a communication carrier. Typically, this involves activating the transceiver of the unit, and “listening” for a signal from a local communication carrier system that provides identification and other information regarding the communication carrier system. Once such information is received by the unit, or the unit is unsuccessful in receiving such information from a local communication carrier, the procedure continues to Step  56 . In Step  56 , the station identification number (SID) associated with the portable communication unit is compared to the SID received from the local communication carrier system. If the two SID&#39;s are the same, the portable communication unit is in the area of its “Home” communication carrier, and the procedure continues to Step  58 . 
   In accordance with an important feature of the present invention, the portable communication unit operates based upon discrete and individually allocable blocks of “airtime.” According to one embodiment of the invention, there is provided two different types of airtime blocks, home time blocks and roam time blocks. Generally, home time blocks will allow the portable communication unit to engage in a communication session when in its home service area, while roam time blocks will be required for the unit to engage in a communication session when the unit is outside of the home service area. The station identification number (SID) is the parameter which is utilized to determine if the portable communication unit is within its home service area, is in a service area outside of its home service area, or if the unit is in an area where no service is available. 
   If, in Step  56 , the comparison of the SID associated with the portable communication unit and the SID received from the communication carrier, if any, are not the same, the procedure continues to Step  62  to determine if the unit should operate in “Roam” mode, i.e., the unit is outside of its home service area. In Step  62 , the SID information associated with the communication carrier is checked to determine if it falls within a predetermined authorized range. If the SID information associated with the communication carrier falls outside of a predetermined acceptable range of values, or the portable communication unit has not received SID information from any available communication carrier from Step  54 , then the procedure continues to Step  64  to display a message indicating that the unit is presently in an area where no service is available. Otherwise, in Step  62 , if the SID information associated with a communication carrier is determined to be valid, i.e., within a predetermined acceptable range, this is an indication that the unit is in a “Roam” service area, and the procedure continues to Step  66 . If, in Step  66 , the portable communication unit determines that roam time is available from a roam time block of airtime, the procedure continues to Step  60  to complete the registration process. Otherwise, in Step  66 , if there is no roam time available from a roam time block, the procedure continues to Step  68  where a message is displayed indicating that the portable communication unit does not have any airtime left to complete the registration process and provide service to the user. 
   If in Step  58 , the portable communication unit determines that home time is available from a home time block of airtime, the procedure continues to Step  60  to complete the registration process. Otherwise, in Step  58 , if there is no home time available from a home time block, the procedure continues to Step  68  where a message is displayed indicating that the portable communication unit does not have any airtime left to complete the registration process and provide service to the user. 
   According to a preferred embodiment of the invention, the user of the portable communication unit will, even if no home time or roam time airtime is available, be able to engage in a communication session with at least one predetermined number, such as an emergency telephone number. When the procedure has reached Step  68  and the “Need Credit” message is displayed, and digits from the keypad of the unit are depressed, the procedure will continue to Step  69  to determine if the sequence of digits consists of a predetermined code for activating the routine to add additional home or roam time credit. If, in Step  69 , the particular predetermined sequence is received to activate the add credit routine, which may be comprised of, perhaps, the menu key  22  ( FIG. 1 ) in conjunction with another numeric key, the procedure continues to Step  71  where the Add Credit Routine is performed. Otherwise, in Step  69 , the procedure continues to Step  70  where the digits entered are analyzed to determine if they correspond to an emergency or other pre-authorized number which does not require airtime from home blocks or roam blocks to process the call. For example, according to a preferred embodiment of the invention, the portable communication unit of the present invention will allow at least one predetermined emergency number to be processed, which may be activated, for example, by pressing a dedicated “hot” key  24  ( FIG. 1 ), even if no home block or roam block airtime is available. This feature of the invention ensures the user that, even though all home time and roam time airtime might be used up, the portable communication unit will still be able to provide the user with the ability to process a call to an emergency number and contact emergency personnel, since such a portable communication unit is widely recognized as an indispensable device to alert authorities in the event of an emergency situation. If such a pre-authorized emergency number is entered, the procedure jumps from Step  70  to Step  90  to a routine that processes the call. As should be evident, in addition to an emergency telephone number, the portable communication unit of the present invention may also be programmed to recognize other pre-authorized numbers, such as the telephone number corresponding to the dealer so that the user can communicate with the dealer to obtain the information required to add another home time or roam time block of airtime. 
   In Step  60  the portable communication unit executes a conventional registration process with the portable communication service carrier, where, for example, information is exchanged regarding the type and identification of the portable communication unit and other conventional verification routines are performed. If registration is successful, the procedure continues to Step  80  where a “Ready” message is displayed and the unit remains ready to handle an incoming call or process a request to make an outgoing call. Otherwise, in Step  60 , if registration is unsuccessful, the procedure loops back to Step  64  where a “No Service” message is displayed and the procedure continues back to the initialization step in another attempt at registration. 
   After the portable communication unit has been registered, if signals are received from the communication carrier indicating that an incoming call is available, the process jumps to Step  90  where the incoming call is processed in accordance with a relatively conventional portable communication call processing routine. Otherwise, if the user enters a number on the keypad, the procedure continues from Step  84  to Step  86  where information relating to the digits depressed on the keypad are received until terminated when the “Send” key is activated. Next, in Step  88 , one of a possible series of checks are performed to determine if the number entered by the user is authorized. As will be described in greater detail, an important feature of the invention is that the portable communication unit may be provided with an access restriction profile, which limits the area for which long distance and even local calls may be placed. 
   In Step  88 , the length of the number entered is checked; if the number is less than 7 digits, the procedure continues to Step  90 , otherwise the procedure continues to Step  92  for further checking. A number which is less than 7 digits, will be processed since it likely pertains to an emergency number or possibly one of several special “star” numbers which may be offered by the communication carrier such as, for example, *611 for connection to an operator or special service that provides traffic or directional information. If the number is not less than 7 digits, the procedure continues to Step  92  where the number is checked to determine if it is a toll-free “800” number; if it is an “800” number, the procedure continues to Step  90  to process the call, otherwise, if the number is not an “800” number, the procedure continues to Step  94  for additional checking. In Step  94  the number entered is compared to the Mobile Identification Number (MIN) unique to and associated with the portable communication unit. If the area code and the dialing prefix, i.e., the first 3 of 7 numbers of a local number, of the MIN is the same as the number entered by the user, the number entered is a local number and the routine continues to Step  90  to process the call. Otherwise, in Step  94 , if the area code and the dialing prefix of the MIN differs from that of the number entered by the user, the procedure continues to Step  96  for further checking. In Step  96 , the number entered by the user is compared to a set of authorized pairs of area codes and dialing prefixes in order to determine if the number entered by the user is authorized. If, in Step  96 , the comparison indicates that the number entered by the user is authorized, the procedure continues to Step  90  where the call is processed, otherwise the number entered is not authorized and the procedure continues to Step  98  where a message, such as “Access Restriction” is displayed indicating that the number entered by the user is not authorized. 
   Except for the out-of-time step indicated in Step  100 , the portable communication unit of the invention operates in Step  90  substantially as a conventional portable communication unit to process the call which has either been entered by the user or received by the carrier for connection to and communication with the portable communication unit. When a communication session has been established in Step  90 , home time or roam time airtime, whichever is appropriate, will be decremented in accordance with the connection time for the communication. For example, if the portable communication unit is powered up, the unit determines, from signals received from the communication carrier that the unit is in a “home” area, registration is successful and the user has entered a local 7 digit number, in Step  90  after the appropriate connection is made, units of time corresponding to the duration of the call are decremented from the current home time block of airtime. During the processing of the call and any communication during Step  90 , the home time or roam time airtime, whichever is appropriate, is decremented, and, in Step  100 , the current block of airtime is checked to determine if the available airtime has been completely depleted. As should be evident, after a call is terminated in Step  90 , the routine jumps back, for example, to Step  52 , or possibly Step  80 , where the portable communication unit remains “idle” until another number is entered by the user or an incoming call is received for the portable communication unit. If, however, the appropriate home time or roam time airtime is completely used up during a communication session in Step  90 , the procedure, in Step  100 , will usually branch back to Step  68 , resulting in a termination of the call and display of the “Need Credit” message. 
   As should be evident, however, in the event a call to an emergency number is being processed, which might have been initiated, for example, in Step  70 , an “emergency call” flag will be set. During Step  100 , the routine determines that there is no more airtime for processing the call, the “emergency call” flag should be checked to determine if the number corresponding to the call is one of the pre-approved or emergency numbers allowed to be processed when all airtime in the unit has been used up. Thus, in Step  100 , if the “emergency call” flag is set, an emergency call is presently being processed and the routine branches back to Step  90  to complete the emergency call, regardless of whether there is sufficient airtime to process the call. Otherwise, in Step  100 , if there is no airtime remaining and the “emergency call” flag is not set, a regular call is presently being processed, and therefore the call is terminated and the routine branches back to Step  68  to display the “Need Credit” message. 
   Turning now to  FIG. 3 , there is illustrated a flow diagram of a routine according to one embodiment of the invention for adding an additional block of either home time or roam time airtime to the portable communication unit of the present invention. This routine might be entered, for example, by the user pressing the menu key  22  ( FIG. 1 ) in combination with one or more predetermined numeric keys. The Add Credit Routine will typically be entered when the user enters a special keystroke sequence indicating that the user desires to access the Add Credit Routine. Alternatively, the portable communication unit of the present invention may be provided with an additional dedicated key for the user to activate in order to enter the Add Credit Routine. 
   The Add Credit Routine illustrated in connection with  FIG. 3  relates to a routine where a single block of either home time or roam time airtime is activated when the appropriate code is entered into the portable communication unit. According to an alternative embodiment of the invention, which could be rather readily implemented by a modification to the software that operates the Add Credit Routine, more than one block of airtime could be added with a single code. In other words, the code entered could be checked by the routine to determine if it is one of several different codes, where each of the codes relates to an authorization to activate one or more blocks of airtime. Such a modification would, of course, likely result in a corresponding modification to the Code Generation Routine illustrated in  FIG. 5 . Thus, by such a modified routine, a user could obtain from the dealer a code that would activate two or more blocks of either home time or roam time airtime. The routine illustrated in  FIG. 3 , however, is limited to an implementation where a separate code must be entered in order to activate each additional block of airtime. 
   Referring to  FIG. 3 , after the Add Credit Routine is entered in Step  71 , the procedure continues to Step  150  to determine if the user desires to add home time or roam time airtime. Preferably, the user will be prompted with an appropriate message in the display  20  ( FIG. 1 ) of the portable communication unit. If, in Step  150 , the user indicates that home time airtime is desired, the procedure continues to Step  152 , where the user is prompted to enter the next unique code to allocate another block of home time airtime. According to a preferred embodiment of the invention, the user will only be able to receive such a code by contacting the portable communication unit dealer who will authorize and pre-approve the user for another block of home time airtime. In Step  154 , the code entered by the user in Step  152  is compared to the next code generated by the portable communication unit to determine if the user has entered the correct code. As will be described in greater detail, the portable communication unit includes a routine to generate, preferably through an encryption technique, a sequence of such codes based upon a plurality of various parameters, some of which relate to information unique to the particular portable communication unit. In Step  154 , if the code entered by the user is the same as the next home time code generated by the portable communication unit, the procedure continues to Step  156  where another block of home time is allocated and the home time block counter is incremented. Next in Step  158 , the procedure returns to the point where it was called. 
   If, however, in Step  154 , the code entered by the user differs from the next home time code generated by the portable communication unit, the procedure branches to Step  160  where a counter relating to unsuccessful attempts to enter another airtime code is incremented. Next, in Step  162 , a message is displayed indicating that the home time code entered by the user in Step  152  was incorrect. In Step  164  the unsuccessful attempt counter is checked to determine if it exceeds a predetermined maximum number. According to this important and preferred feature of the invention, if the user enters a wrong airtime code more times than the predetermined maximum number, the portable communication unit of the invention will automatically disable and operation can only be enabled once again when the unit is returned to the dealer, or possibly the manufacturer, and reset. This important feature of the invention serves as a useful deterrent to theft or misuse of the portable communication unit, since any authorized user that attempts to repetitively and unsuccessfully “guess” and enter codes for additional airtime will be denied any further unauthorized use of the unit. On the other hand, if in Step  164  the user enters an incorrect home time code and the number of attempts is less than the predetermined maximum, the procedure, in Step  170 , loops back to the Add Credit Routine for the user to attempt to enter the correct home time code once again. 
   As can be seen from  FIG. 3 , if, in Step  150 , the user indicates that it is desired to enter additional roam time rather than additional home time airtime, the procedure branches to Step  172  where, in Steps  174  through Steps  190 , there is provided a routine for checking and entering codes for additional roam time airtime similar to the routine set for in Steps  154  through Steps  170  with respect to adding additional home time airtime. 
   Turning now to  FIG. 4A , there is illustrated, by way of example, a number of parameters used by the various operating routines of the portable communication unit in accordance with the present invention. As can be seen, the unit stores a plurality of different parameters required to properly operate and identify the unit. These parameters include, a number of relatively conventional parameters, such as the Electronic Serial Number (ESN), the Mobile Identification Number (MIN), and the Station IDentification (SID) associated with the particular portable communication unit. The ESN, is similar to a conventional serial number, in that it is a number unique to each portable communication unit and is typically assigned to the unit when the unit is manufactured. A full ESN is typically, when expressed in decimal or base 10 notation, 11 digits in length. The first three digits, correspond to and identify the manufacturer of the portable communication unit, the next two digits are reserved or unused, and the remaining 6 digits are a unique identifier for the unit. According to present embodiment of the invention, 5 of the last 6 digits of a full ESN are used. With reference to the ESN illustrated in  FIG. 4A , the manufacturer identification portion of the ESN is “183”, while the unit identifier is “052139”, and the portion utilized by the processing routine of the present invention is the last 5 digits, or “52139.” 
   The MIN, like the ESN, is also commonly unique to the particular portable communication unit and represents, essentially, the phone number associated with the unit. Usually, the MIN is ten numeric digits in length, and includes a three digit area code, a three digit dialing prefix and four additional digits. With reference to the MIN illustrated in  FIG. 4A , the area code, represented by the first 3 digits, is “217”, the dialing prefix, represented by the next 3 digits, is “357”, while the local number, which is comprised of the dialing prefix and the last 4 digits, is “3572308.” The SID, typically a five digit numeric code, identifies the “home” carrier station for the portable communication unit. The SID illustrated in  FIG. 4A  is “21503.” Since many portable communication units may be assigned to the same carrier station, usually based upon geographic location of the dealer or the customer with respect to the nearest portable communication carrier station, the SID is almost always a code that is not unique to the particular portable communication unit. 
   The portable communication unit of the present invention also includes several parameters which are not found in conventional units, including a Dealer Password, Current Roam Block, Roam Time, Current Home Block, Home Time, and a set of Toll Restrictor Codes. The Dealer Password, comprised of, perhaps, 4 numeric digits as illustrated, is a code that identifies and is unique to a particular dealer for the portable communication unit. The Current Roam Block is an index parameter that represents the current number of the roam block. Thus, each time a roam block of airtime is used up, this parameter is incremented. With reference to  FIG. 4A , the Current Roam Block is block number  2 , indicating that 1 block of roam time airtime has already been used. The Roam Time is a parameter that represents the current amount of airtime available for operating the unit in roam time mode. Illustrated by way of example in  FIG. 4A  and assuming that the airtime parameter as illustrated in seconds, there is presently 722 seconds of roam time available in the current roam time block of airtime. The parameters Current Home Block and Home Time have a similar meaning in the context of home time. 
   It should be noted that the airtime parameter is illustrated in  FIG. 4A  in units of seconds only by way of example, and the airtime parameter may be based upon different unit. Furthermore, it should be noted that according to an alternative embodiment of the invention, it is possible that, when there is no additional roam time airtime remaining, home time airtime could be utilized to operate the unit while in a roam area. In this case, the home time airtime, which typically is less expensive than the roam time airtime, would preferably be decremented at a greater rate than if the unit were operating in a home area, in order to reflect the difference in the cost between the two rates. Similarly, when there is no additional home time airtime remaining, roam time airtime could be utilized to operate the unit while in a home area. In this case, the roam time airtime, which typically is more expensive than the home time airtime, would preferably be decremented at a lesser rate than if the unit were operating in a roam area, in order to reflect the difference in the cost between the two rates. 
   The set of Toll Restrictor Codes represents a number of different codes that specify the allowable combination of area codes and dialing prefixes that the portable communication unit may use for operation. Illustrated by way of example in  FIG. 4A , the portable communication unit is presently programmed with a set of 4 Toll Restrictor Codes. The first Toll Restrictor Code is “217FFF”, where “217” represents the allowed area code and “FFF” represents the associated allowed dialing prefix. As represented in  FIG. 4A , a Toll Restrictor Code symbol of “F” refers to a type of “wild card” digit, where any digit will be allowed. Thus, with respect to the first Toll Restrictor Code, any dialing prefix will be allowed, as long as the area code is “217.” The second Toll Restrictor Code, “312245”, does not contain any “wild card” symbols and therefore will allow only the combination of an area code of “312” and a dialing prefix of “245.” The third Toll Restrictor Code is “414FFF” and is similar to the first Toll Restrictor Code in that it allows any dialing prefix, but requires an area code of “414”. The fourth Toll Restrictor Code, “7088FF”, indicates an area code of “708” and a dialing prefix with a first digit of “8” followed by any 2 other digits. Thus, with respect to the fourth Toll Restrictor Code, the portable communication unit would be allowed to initiate a communication session with a number “7088234221”, and would be allowed to initiate a communication session with a number “7088437904”, but would be restricted from initiating a communication session with a number “7087337454.” As can be seen from the foregoing examples, the set of Toll Restrictor Codes may be viewed as essentially a set of “masks” where an exact digit match must be present in a position where a specific digit is specified and any digit may be present where the “wild card” symbol of “F” is specified. Preferably, the portable communication unit will have sufficient memory to accommodate many different Toll Restrictor Codes in order to provide a wide variety of allowable numeric combinations with which the portable communication unit may initiate a communication session. The Toll Restrictor Codes, of course, do not necessarily restrict the portable communication unit from establishing a communication session with a number not allowed, as long as the session is initiated by the other party. 
   Some of the various parameters illustrated in  FIG. 4A , will be set by the dealer when the portable communication unit is delivered to the customer. In particular, the MIN, SID, Dealer Password and Toll Restrictor Codes will typically be set by the dealer, and preferably can only be changed when the unit is given back to the dealer. The ESN, on the other hand, will typically be set by the manufacturer when the portable communication unit is manufactured. Thus, those parameters are generally not changed while the unit is in the possession of the user, and therefore remain the same until the unit is delivered back to the dealer. The other parameters, namely, the Current Roam Block, Roam Time, Current Home Block, and Home Time may be changed indirectly by the user through operation of the Add Airtime Routine or by using home time or roam time airtime through operation of the portable communication unit. 
   Illustrated in  FIG. 4B  is an example of how the various parameters used by the portable communication unit, as well as a set of randomly selected but predetermined numbers, might be stored in different tables in the memory of the portable communication unit in order to be utilized to generate the sequence of unique codes or “keys” to be checked against the numbers entered by a user to open additional blocks of airtime. As illustrated, there are provided three different tables of various lengths. Table1 stores a unique and random sequence of numbers used, in combination with numbers from the other tables, to generate the home time and roam time codes to be checked against the codes entered by a user to add additional airtime. The unique and preferably random sequence of numbers stored in Table1 may be specific to the particular portable communication unit in which they are stored. In any event, the sequence of numbers stored in Table1 does not change while the unit is in the possession of the user. Table2 stores the various digits comprising the ten digit mobile identification number (MIN) associated with the portable communication unit, while Table3 stores the various digits associated with the four digit electronic serial number (ESN), the five digit station identifier (SID), and the four digit dealer password. 
   Turning now to  FIG. 5 , there is illustrated, by way of example, a routine for generating a set of codes corresponding to “keys” for opening additional home time blocks of airtime, based upon information set forth in the tables Table1, Table2 and Table3 as illustrated in  FIG. 4 . Although particular routine illustrated in  FIG. 5  generates a set of home time codes, a set of roam time codes may be generated with a nearly identical routine. According to the example, there is a maximum number of 36 different five-digit codes, thus a maximum of 36 blocks of airtime, for either home time or roam time airtime may be allocated. After all the blocks are allocated the portable communication unit must be returned to the dealer to be reactivated. As should be evident, however, a different code generating routine could be implemented whereby an essentially endless number of codes may be generated. The Code Generation Routine illustrated in  FIG. 5 , operates by utilizing the information set forth in the tables, at least some of which is unique to the particular portable communication unit, and generates digit by digit the sequence of 36 roam time and home time codes. A digit generation routine, GenDigit generates a digit based upon three numeric parameters supplied. The GenDigit routine is preferably an encryption-type that generates a unique digit when supplied with numeric parameters. 
   After the Code Generation Routine is entered in Step  300 , the procedure continues to Step  305  where a set of pointers or index variables, used to sequence through the various number stored in the different tables, are initialized such that they refer or index to the beginning number in each of the tables. The variables T1_POINTER, T2_POINTER and T3_POINTER are used as indexes to the various numbers stored, respectively, in Table1, Table2 and Table3. Next, in Step  310 . the variable N_COUNT, which is used in the Code Generation Routine as a counter represetning the curfrent airtime block number, is cleared by setting it equal to 0. Similarly, in Step  315 , the variable D_COUNT, which is used in the Code Generation Routine as a counter representing the current code digit for a particular airtime block number, is cleared by setting it equal to 0. Next, in Step  320 , a unique digit is generated and assigned to the variable TEMP when the GenDigit is called and supplied with the table numbers indexed by the index variables. Thus, when the GenDigit routine is called for the first time, it will generate a number which is used as the first digit of the first code for the home time airtime block. 
   After a unique encrypted digit is generated from the GenDigit routine in Step  320 , the procedure continues to Step  325  where the table pointers are incremented in preparation for generation of the next digit. Next, in Step  330 , the pointer T1_POINTER is checked to determine if it exceeds the length of Table1; if it does, the procedure continues to Step  335  where T1_POINTER is reset to 0, otherwise the procedure continues to Step  340 . Similarly, in Step  340 , the pointer T2_POINTER is checked to determine if it exceeds the length of Table2; if it does, the procedure continues to Step  345  where the T2_POINTER is reset to 0, otherwise the procedure continues to Step  350 . In Steps  350  and  355  the T3_POINTER is similarly checked and reset to 0 if it exceeds the length of Table3. Next, in Step  360 , the digit generated by the GenDigit routine is assigned to the current digit of the current home time code number by setting the two-dimensional HOMECODE array, indexed by the N_COUNT and D_COUNT index variables, to the value of the variable TEMP generated by the GenDigit routine. Next, in Step  365 , the digit counter variable, D_COUNT is incremented, and in Step  370  is checked to determine if all the digits for the current home time code have been generated. If all the digits for the current home time code have not yet been generated, the procedure branches back to Step  320  where another digit is generated. Otherwise, if all the digits for the current home time code have been generated, the procedure branches to Step  375  where the code number counter, N_COUNT is incremented. In Step  380 , the value of N_COUNT is checked to determine if all the home time code numbers have been generated. If N_COUNT equals 37, all the home time code number have been generated and the routine returns in Step  385 . If, in Step  380 , not all the home time code numbers have yet been generated, the routine branches back to Step  315  where the digit counter D_COUNT is cleared in preparation for generating the first digit of the next home time code. 
   A similar routine to that illustrated in  FIG. 5  may be used to generate codes used to activate additional blocks of roam time airtime. 
   As should be apparent from the foregoing description of the Code Generation Routine, the dealer, when requested by the user for an additional block of home time airtime, runs essentially an identical routine in order to provide the next authorized code to the user. Thus, all of the parameters associated with the particular portable communication unit of interest are preferably electronically stored and may be loaded and utilized in connection with the Code Generation Routine to generate the list of home time codes. The significant difference between the operation of the Code Generation Routine performed by the dealer and the routine performed by the portable communication unit is that the routine used by the dealer may be used to display, print out or otherwise output the codes, while the routine used by the portable communication unit itself, does not display or output the codes, but is limited to using the codes generated in order to verify that the user has entered the correct next code to activate the next block of airtime. By this process, the dealer may be assured that the user can only use additional airtime when authorized by the dealer, through providing the user with appropriate code to activate additional airtime. 
   According to an alternative arrangement, the Code Generation Routine might be run by the manufacturer of the portable communication unit, and the dealer, rather than performing the Code Generation Routine to determine the appropriate airtime codes, will be provided with a list of home time and roam time airtime codes generated by the Code Generation Routine in connection with each particular portable communication unit supplied to the dealer. 
   According to another alternative embodiment of the invention, an additional block of airtime for the portable communication unit might be activated without the need for any manual intervention by the user. For example, the portable communication carrier station could transmit an activation signal to the portable communication unit while the unit is in the process of registering with the station. In response to receiving such an activation signal upon registration, the unit could automatically activate an additional block of airtime. Such an activation signal would, of course, preferably only be transmitted when the dealer, or possibly the portable communication service provider, is assured that the user has adequate credit or has pre-paid for the additional block of airtime. Thus, according to such an alternative embodiment, the user would receive a portable communication unit from a dealer of portable communication equipment. Initially, the unit would be activated with an initial block of airtime, and the user would contact the dealer when the airtime is exhausted. When the dealer is satisfied that the user has sufficient credit for the additional block of airtime, the dealer will notify the portable communication carrier to transmit an activation signal when the portable communication unit attempts to register with the portable communication carrier station. Alternatively, such an activation signal could be transmitted at any time that the portable communication unit is engaged in a communication session with the portable communication carrier station. 
   According to yet another, although likely less desirable alternative arrangement, a portable communication unit in accordance with the invention could be configured such that an additional block of airtime could be activated manually by the dealer, such that the user would need to return to unit to the dealer when an additional block of airtime is desired. Such an embodiment might be useful in an environment where the unit is rented on a short term basis, for example, in conjunction with rented an automobile, perhaps, or other piece of equipment, where it is intended that the user will be returning the unit to the dealer within a relatively short period.