Call-back method in response to emergency call originating from cellular radiotelephone

An emergency call originates from a cellular radiotelephone (20) and communicates through one of A-side and B-side cellular systems. Upon receipt of the call, the cellular radiotelephone (20) is assigned a temporary substitute Mobile Identification Number (MIN) (100) by the receiving A-side or B-side cellular system MTSO (33). Upon termination of the emergency call, a call-back to the cellular radiotelephone (20) is routed to both of the A-side and B-side MTSOs (100), using the substitute MIN (100), by the Public Switched Telecommunications Network (36). Both A-side and B-side MTSOs (100) issue a page order message to cellular radiotelephone (20) by transmitting to the MIN (74). Paging order confirmation messages (78), conveying the MIN (74), are evaluated for a match with a unique Electronic Serial Number (76).

TECHNICAL FIELD OF THE INVENTION 
The present invention relates generally to cellular radiotelephones. More 
specifically, the present invention relates to a cellular radiotelephone 
connection with a Public Safety Answering Position (PSAP). 
BACKGROUND OF THE INVENTION 
Cellular radiotelephones have become increasingly popular for many 
different reasons, including their potential usage in case of emergency. 
Cellular radiotelephones adequately meet the needs of emergency situations 
because they can be carried with a user to be readily available whenever 
and wherever needed. However, cellular radiotelephones have a distinct 
disadvantage from conventional landline telephones in that they may not 
provide sufficient information in which to reliably perform a return call. 
When an emergency call is placed from a cellular radiotelephone, the call 
is routed to a public safety answering position (PSAP). If the two parties 
are unexpectedly disconnected or if the call has terminated but additional 
information is needed, a "call-back" may be desired. A call-back is a 
return call made by the PSAP directed to the cellular radiotelephone from 
which the emergency call originates. 
In accordance with conventional methodologies, when the cellular 
radiotelephone is a subscriber within it's own service area, a call-back 
is a local call to the radiotelephone Mobile Identity Number (MIN) for the 
cellular radiotelephone making the original emergency call. Likewise, the 
subscriber can be a roamer who has activated a "call follow" service. In 
conventional systems, a call follow service assigns a temporary local 
phone number to the roaming cellular radiotelephone and sends a call 
forward order to the roamer's service area. When an emergency call is 
placed by the roaming cellular radiotelephone, the PSAP receives the local 
phone number as the number from where the emergency call originated. 
During call-back, the PSAP directs the call to the local phone number. The 
roaming area MTSO receives the call-back to the local phone number and 
translates the temporary local phone number into the roaming cellular 
radiotelephone's MIN. The call-back is then directed to the roaming 
cellular radiotelephone. 
If an emergency call is originated by a subscriber who is roaming and has 
not activated a call follow service, the PSAP will receive a MIN that is 
invalid in the roamer's current service area. In accordance with 
conventional methodologies, a call-back will fail because a call-back to 
an invalid MIN will not be placed through a MTSO. 
In these situations, a call-back is not necessarily successful. A further 
complication arises if the cellular radiotelephone switches to the 
non-preferred A-side or B-side system after the emergency call but prior 
to the attempted call-back. This can occur if the cellular radiotelephone 
moves in an area were the signal strength of the non-preferred system is 
significantly greater than that of the preferred system, or if the 
preferred system becomes unavailable. In accordance with conventional 
methodologies, call-backs are directed through the preferred A-side or 
B-side system for that cellular radiotelephone, hence the call-back will 
fail because the cellular radiotelephone is no longer monitoring that 
cellular system. 
If a call-back is not received by the cellular radiotelephone, the PSAP 
could possibly attempt the call-back over the other one of the local 
A-side or B-side systems to ameliorate the situation where the cellular 
radiotelephone switches systems. In accordance with current methodologies, 
call-back attempts over both A-side and B-side MTSOs would be performed 
sequentially thereby costing valuable time. Moreover, if the caller is 
roaming and the caller's home service area has no roaming agreement with 
either system in a roaming area, the call-back attempt will still fail. 
Additional problems arise with "unauthorized" cellular radiotelephones. 
Unauthorized cellular radiotelephones are those cellular radiotelephones 
that do not have a valid, unique MIN. While normal calls cannot originate 
from an unauthorized cellular radiotelephone, emergency calls are an 
exception. Emergency calls are routed to the PSAP without confirming the 
validity of the cellular radiotelephone to be using the system. However, 
current methodologies do not allow for call-backs to be successfully made 
to these unauthorized cellular radiotelephones. Unauthorized cellular 
radiotelephones include "non-subscribers" and "inactive" cellular 
radiotelephones. 
A non-subscriber is a cellular radiotelephone that was once a subscriber 
with an assigned MIN, but is no longer a paying customer. The MIN may 
remain programmed in the cellular radiotelephone, however it is invalid 
because the MIN is no longer assigned to that cellular radiotelephone by 
the home service area MTSO. A normal call can only be connected if the MIN 
and the radiotelephone Electronic Serial Number (ESN) match the assigned 
MIN/ESN, as reported by the home service area MTSO. Unless both numbers 
match, the home service area for that MIN will not steer the call to this 
unauthorized cellular radiotelephone. 
An inactive cellular radiotelephone has never been a subscriber to a 
cellular service, therefore it may not have a unique MIN. A call-back 
cannot be performed because this cellular radiotelephone has no dialable 
telephone number. 
Given these examples, if an emergency call originating from a cellular 
radiotelephone is disconnected, conventional Mobile Telephone Switching 
Offices (MTSOs) may be unable to set up a call-back from the PSAP to the 
cellular radiotelephone. This inability to return a call is a serious 
problem in an emergency situation. 
SUMMARY OF THE INVENTION 
Accordingly, it is an advantage of the present invention to provide an 
improved method of call-back following an emergency call from a cellular 
radiotelephone. 
Another advantage of this invention is to provide this call-back method for 
subscribers of cellular radiotelephones outside of their service areas and 
users of cellular radiotelephones that are not subscribers to a cellular 
service. 
Another advantage of this invention is to provide this call-back method 
concurrently through both A-side and B-side cellular systems to increase 
the probability of a rapid and successful call-back. 
Another advantage of this invention is to implement this process without 
requiring changes to existing cellular radiotelephones. 
The above and other advantages of the present invention are carried out by 
a method in which an MTSO can provide call-back to a cellular 
radiotelephone following an emergency call. The method calls for detecting 
an emergency call from a cellular radiotelephone to a PSAP and assigning 
the radiotelephone a substitute MIN. The MTSO then enables the call. 
Following termination of the call, the MTSO detects a call-back to the 
substitute MIN. The MTSO then pages the cellular radiotelephone assigned 
that substitute MIN.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 shows a layout diagram of a cellular system environment 18 within 
which an emergency call from a cellular radiotelephone 20 may originate. 
In cellular system environment 18, the same cellular radiotelephone 20 is 
depicted at two different points in time in two different areas (discussed 
below). Although FIG. 1 shows a single cellular radiotelephone 20, any 
number of cellular radiotelephones 20 may operate in this and other 
similar environments. Cellular radiotelephone 20 can be configured as a 
conventional portable unit which is easily carried from place to place, 
thus able to freely move about within environment 18. 
A local area 22 contains both local A-side region 24 and local B-side 
region 26 for cellular radiotelephone 20. Local A-side region 24 and local 
B-side region 26 are supported by a plurality of "A" land stations 28 and 
"B" land stations 30. "A" land stations 28 and "B" land stations 30 cover 
diverse cells or areas of geography. Local A-side region 24 and local 
B-side region 26 may, and typically do, cover the same geographical areas 
and have a plurality of land stations 49. 
The overlapping of local A-side region 24 and local B-side region 26 
results from FCC rules which are intended to encourage competition in the 
provision of cellular radiotelephone services. No interference between 
communication taking place in overlapping cells occurs because A-side 
cellular systems operate only on one set of frequencies, or channels, 
assigned by the FCC, and B-side cellular systems operate only on an 
entirely different set of channels. 
FIG. 1 illustrates only a minor overlap between local A-side region 24 and 
local B-side region 26 for clarity of illustration. However, those skilled 
in the art will appreciate that extensive overlap exists in most 
metropolitan areas. While extensive overlap between A-side and B-side 
cellular systems typically exists, nothing in the present invention 
requires any overlap to be present, and A-side and B-side cellular systems 
may both fail to cover some areas. 
In FIG. 1, "A" land stations 28 and "B" land stations 30 are connected to 
local A-side and B-side system mobile telephone switching offices (MTSOs), 
32 and 34 respectively, via conventional land lines or microwave links. In 
turn, local A-side and B-side MTSOs 32 and 34 connect to the public 
switched telephone network (PSTN) 36. Additionally a local public safety 
answering position (PSAP) 38 is linked to PSTN 36 via conventional 
landlines. MTSOs 33 in cellular system environment 18, which include local 
A-side and B-side MTSOs 32 and 34 respectively, control and supervise the 
connection of cellular radiotelephone 20 to public switched telephone 
network (PSTN) 36. 
Typically, service to A-side cellular region 24 is provided by an entirely 
separate organization from that which provides B-side cellular region 26. 
In accordance with conventional advanced mobile phone services (AMPS) 
cellular telephony methodology, cellular radiotelephones are subscribers 
to one of the A-side and B-side systems. These radiotelephones are 
programmed to prefer engaging in communications with the specific home 
system and system type (either A or B) to which they subscribe. 
FIG. 2 illustrates four signaling paths used in the preferred embodiment 
for communication between land stations 49 and cellular radiotelephone 20. 
A forward control channel 58 and a reverse control channel 60 are used to 
set up calls and manage cellular radiotelephones 20 on a cellular system. 
A forward voice channel 62 and a reverse voice channel 64 are used to 
communicate voice and other user-provided information for managing the 
calls. Data is transmitted on these voice channels before, after, and 
during a call. 
FIG. 3 shows a data format diagram depicting a portion of data transmitted 
over forward control channel 58. Forward control channel 58 is used by 
land station 49 (FIG. 1) to convey information from MTSO 33 (FIG. 1) to 
cellular radiotelephone 20. In response to orders sent from MTSOs 33 (FIG. 
1), land stations 49 send a continuous stream of broadcast data over 
forward control channel 58. This broadcast data typically includes 
busy/idle bits 66, a bit sync 68, and a word synch 70. Each frame of the 
data stream over forward control channel 58, contains bit sync 68 and word 
sync 70 to enable cellular radiotelephone 20 to obtain synchronization. In 
accordance with conventional AMPS cellular telephony practice, busy/idle 
bits 66 are sent at the beginning of every bit sync sequence, word sync 
sequence, first repeat of a word and every ten message bits thereafter to 
indicate the current busy/idle status of the corresponding reverse control 
channel 60 (FIG. 2). Information is sent in forty bit words and can take 
the form of a mobile station control message 72. 
Mobile station control message 72 is sent to tell cellular radiotelephone 
20 what is required of it. The message contains a mobile identification 
number (MIN) (discussed below) of cellular radiotelephone 20. 
FIG. 4 shows a data format diagram depicting a portion of data transmitted 
over reverse control channel 60. Reverse control channel 60 is used by 
cellular radiotelephone 20 to convey data to land station 49 and MTSO 33 
(FIG. 1). If cellular radiotelephone 20 is performing a registration, 
cellular radiotelephone 20 sends three words containing a radiotelephone 
MIN, a radiotelephone electronic serial number (ESN), and other data. If 
an emergency call or other call is initiated by cellular radiotelephone 
20, then an additional two to four words are sent with the requested 
number. 
An activated cellular radiotelephone 20, is programmed with a unique mobile 
identification number (MIN) 74. MIN 74 is a 34 bit binary number derived 
from a ten digit telephone number. An inactive cellular radiotelephone 20 
is typically programmed from the manufacturer with a null MIN 74. The ten 
digits of null MIN 74 are typically all zeroes, thus making MIN 74 
non-unique with respect to other inactive cellular radiotelephones. 
Cellular radiotelephone 20 also has an electronic serial number (ESN) 76 
which is a unique number given to cellular radiotelephone 20 during 
manufacture. ESN 76 is permanent and usually not reprogrammable, while MIN 
74 is semipermanent and may be reprogrammed as needed. During activation, 
cellular radiotelephone 20 is programmed with MIN 74, system 
identification for preferred local A-side region 24 or local B-side region 
26, and other parameters. Cellular radiotelephone 20 will use this 
information and the permanently programmed ESN 76 to gain access on a 
preferred A-side or B-side system. 
When calls are initiated at cellular radiotelephone 20, the number to be 
called is typically loaded via the keypad. Cellular radiotelephone 20 
checks forward control channel 58 and monitors busy/idle bits 66. When 
busy/idle bits 66 are idle, cellular radiotelephone 20 performs a system 
access. System access is accomplished by sending data over reverse control 
channel 60 for processing. This data contains MIN 74, ESN 76 and the 
called number. For normal calls (i.e. non-emergency calls), processing 
includes confirming that MIN 74 and ESN 76 are valid on the system. When 
confirmed, land station 49 sends mobile station control message 72 (FIG. 
3) to allocate forward and reverse voice channels 62 and 64 (FIG. 2) and 
at the same time sets up the call on forward voice channel 62. Cellular 
radiotelephone 20 checks the data and stores it in memory, then moves to 
reverse voice channel 64 in order to open a conversation path. 
At each given period of time in which cellular radiotelephone 20 is in an 
idle mode, it monitors a forward control channel 58 used by either an 
A-side or B-side cellular system for an incoming call. When an incoming 
call is being directed to cellular radiotelephone 20 through MTSO 33 (FIG. 
1), mobile station control message 72 is transmitted over forward control 
channel 58 to page cellular radiotelephone 20. Cellular radiotelephone 20 
monitors busy/idle bits 66 on forward control channel 58 and when reverse 
control channel 60 is free, performs a system access. System access is 
accomplished by sending data over reverse control channel 60. These data 
include MIN 74, ESN 76, and a paging order confirmation message 78. In 
response to paging order confirmation message 78, busy/idle bits 66 are 
changed to busy and data is transferred to MTSO 33 to confirm MIN 74 and 
ESN 76 validity on the cellular system. When confirmed, land station 49 
sends mobile station control message 72, over forward control channel 58, 
to allocate a voice channel for the conversation. Cellular radiotelephone 
20 checks the data and stores it in memory, then returns to reverse voice 
channel 64 to confirm channel set up. Land station 49 sends an alert order 
from MTSO 33 to cellular radiotelephone 20 over forward voice channel 62. 
Cellular radiotelephone 20 responds by sending a signal confirming 
activation of an alerting signal over reverse voice channel 64. When the 
call is answered, a signaling tone is removed and a conversation path is 
opened. 
For calls directed to the 911 emergency number (i.e. an emergency call) MIN 
74 and ESN 76 are not confirmed for validity on the system, however the 
emergency call directed to PSAP 55 will be enabled. Hence, in the case of 
a desired call-back, cellular radiotelephone 20 could be in any one of 
several possible modes of operation. 
Referring back to FIG. 1, cellular radiotelephone 20 is located in local 
area 22, and could be operating in one of several modes. Cellular 
radiotelephone 20 could be a subscriber with a MIN 74 (FIG. 3) for local 
area 22. In another mode, cellular radiotelephone 20 could be a 
non-subscriber with an invalid MIN 74 for local area 22. In a different 
mode, an emergency call could be originating from an inactive cellular 
radiotelephone 20 with a null MIN 74. While conventional MIN/ESN 
confirmation processes might otherwise prevent a call-back to cellular 
radiotelephone 20 from succeeding in these modes, a call-back process 
(discussed below) insures a successful call-back. 
At another point in time, cellular radiotelephone 20, as denoted by 
cellular radiotelephone 20', has moved outside of local area 22 and is a 
roamer in a remote area 40. 
Remote area 40 is also covered by both A-side and B-side cellular systems 
having coverage regions 42 and 44, respectively. This coverage is 
supported by a plurality of "A" land stations 46 for remote A-side region 
42 and a plurality of "B" land stations 48 for remote B-side region 44. 
"A" land stations 46 and "B" land stations 48 are connected to remote 
A-side and B-side system MTSOs 50 and 52, respectively. Remote A-side 
system and B-side system MTSOs 50 and 52 then link to public switched 
telephone network (PSTN) 36 via landlines or microwave links. Likewise a 
remote public safety answering position (PSAP) 54 is linked to PSTN 36 via 
landlines. 
As a subscriber in local area 22, cellular radiotelephone 20' may have 
activated a call follow service. The call follow service results in 
assignment of a local temporary phone number to cellular radiotelephone 
20, and all calls to cellular radiotelephone 20', being forwarded via a 
call follow communication link 56 to remote A-side or B-side system MTSO, 
50 or 52. Call follow communication link 56 is a landline that allows 
communication between MTSOs 33. Conventional processes direct call-backs 
from remote PSAP 54 to cellular radiotelephone 20' via a number that is 
local for remote area 40. 
When a PSAP 55, whether local PSAP 38 or remote PSAP 54, receives an 
emergency call through PSTN 36, the calling party number is supplied to 
PSAP 55. This calling party number can then be used to make a call-back. 
If the cellular radiotelephone's call follow service has not been 
activated, call follow communication link 56 is not present and remote 
PSAP 54 receives the MIN 74 (FIG. 2) as the calling party number. MIN 74 
is not a local number with respect to remote area 40 and is considered 
invalid. While in conventional call-back processes an invalid MIN would 
prevent a call-back from succeeding, a call-back process (discussed below) 
insures a successful call-back. 
FIG. 5 shows a flowchart of a call-back process 80 for operating a cellular 
radio telecommunication network having an A-side system MTSO, 32 or 50 
(FIG. 1), and a B-side system MTSO, 34 or 52. Call-back process 80 
provides an improved method for enabling a return call to cellular 
radiotelephone 20 (FIG. 1) following an emergency call. Call-back process 
80 occurs in either or both of a subscriber's local area 22 (FIG. 1) or 
remote area 40. 
Each MTSO 33 performing call-back process 80 performs a task 86. Task 86 
monitors for incoming emergency calls. Those skilled in the art realize 
that task 86 can occur in conjunction with other MTSO 33 activities which 
are not relevant to the present invention. For example, MTSO 33 may 
evaluate each call received from cellular radiotelephones to determine if 
the call is an emergency call. MTSO 33 will continue to monitor for 
incoming emergency calls in conjunction with other parallel functions. 
In conjunction with task 86, a query task 88 determines if an emergency 
call from cellular radiotelephone 20 is detected. If no emergency call 
initiated by cellular radiotelephone 20 is detected, program control 
returns to task 86 to continue monitoring for incoming emergency calls. 
At query task 88, when an emergency call is detected from cellular 
radiotelephone 20, the MTSO 33 receiving the emergency call becomes a 
primary MTSO 33, and the other MTSO 33 of the A-side and B-side cellular 
system becomes a partner MTSO 33. Primary MTSO 33 launches into primary 
MTSO sub-process 82. For the purposes of this description, a primary MTSO 
is defined to be an MTSO 33 within either the local area 22 or remote area 
40 in which an emergency call from cellular radiotelephone 20 is first 
detected. A partner MTSO is defined to be the other MTSO 33 of the A-side 
and B-side cellular systems within either the local area 22 or remote area 
40 in which the emergency call takes place. Primary MTSO sub-process 82 
includes those tasks performed by the primary MTSO, while partner MTSO 
subprocess 84 includes those tasks performed by the partner MTSO when an 
emergency call is received by the primary MTSO. 
Primary MTSO sub-process 82 begins with a task 90 which performs a 
substitute mobile identification number (MIN) assignment routine. 
FIG. 6 shows a flow chart of substitute MIN routine 90, which is performed 
by primary MTSO 33. A task 92 obtains an identity code for cellular 
radiotelephone 20 over reverse control channel 60 (FIG. 2). The identity 
code includes both MIN 74 and ESN 76 (FIG. 4) for cellular radiotelephone 
20. 
Next, an optional query task 94 determines if all cellular radiotelephones 
20 placing emergency calls are to be assigned substitute MINs. Substitute 
MIN assignment is performed so that detected call-backs are directed 
towards that substitute MIN. In a first embodiment of call-back process 
80, all emergency calls are assigned substitute MINs. This first 
embodiment is a preferred embodiment because it achieves a highly reliable 
call-back method, but at a cost of a large quantity of substitute MINs 
that are required. 
Following optional task 94, if all emergency calls will not be assigned a 
substitute MIN, an optional query task 96 determines if cellular 
radiotelephone 20 is an "unauthorized" radiotelephone. An unauthorized 
radiotelephone is one in which cellular radiotelephone 20 has an invalid 
or a non-unique MIN 74, as discussed previously. At optional query task 
96, if the emergency call is not from an unauthorized cellular 
radiotelephone 20, program control returns to FIG. 5 to set up the 
emergency call. Thus, in a second preferred embodiment of call-back 
process 80, only emergency calls originating from unauthorized cellular 
radiotelephones receive substitute MINs. In this second embodiment of 
call-back process 80, successful call-backs are likely regardless of which 
operational mode cellular radiotelephone 20 may be in. However, there is a 
minor reduction of reliability in successfully making a call-back compared 
to the first embodiment. The reduction in reliability occurs in cases 
where authorized cellular radiotelephones 20 switch systems and begin 
monitoring the partner A-side or B-side system between an emergency call 
and a corresponding call-back. In this situation a call-back directed 
through the primary MTSO 33 may not be received by cellular radiotelephone 
20. However, this second embodiment is a preferred embodiment because the 
minor decrease in reliability has the benefit of decreasing the number of 
required substitute MINs, thus reducing costs. 
Tasks 94 and 96 are considered optional tasks and are provided in FIG. 5 to 
differentiate between the first and second embodiments of the present 
invention. In actuality, either of the above-discussed first and second 
embodiments can be programmed into MTSO 33 prior to the emergency call. In 
the first embodiment of call-back process 80, all emergency calls are 
assigned substitute MINs, therefore tasks 94 and 96 are not relevant. In 
the second embodiment of call-back process 80, only emergency calls 
originating from unauthorized cellular radiotelephones receive substitute 
MINs, therefore task 96 is required to determine which cellular 
radiotelephones will be receiving substitute MINs. 
In routine 90, when either query tasks 94 or 96 are affirmative, program 
control proceeds to a task 98. Task 98 selects an unused substitute MIN 
100 from a list of substitute MINs 102. Primary MTSO 33 then assigns the 
selected substitute MIN 100 to cellular radiotelephone 20, and marks 
substitute MIN 100 as being used on list of substitute MINs 102. 
Substitute MIN 100 is defined as a ten digit dialable North American 
Numbering Plan (NANP) telephone number which is a desirably local number 
relative to primary and partner MTSOs 33. List of substitute MINs 102 is 
accessible by both A-side and B-side system MTSOs 33. The substitute MINs 
100 in list of substitute MINs 102 are assigned by a central office (not 
shown) for PSTN 36 (FIG. 1) and configured such that when a call-back to 
substitute MIN 100 is performed by PSAP 55 (FIG. 1), PSTN 36 initiates a 
conventional multiple routing or ring process. The multiple routing 
process concurrently routes the call-back to both primary and partner 
MTSOs 33. Thus, the call-back will be received concurrently by both 
primary and partner MTSOs 33 for initial processing through both systems. 
As part of the conventional multiple routing process, the central office 
for PSTN 36 will establish a connection through the first MTSO 33 that 
sends back an answer signal. The answer signal indicates that cellular 
radiotelephone 20 has answered the call-back through the first MTSO 33. In 
response to the connection, PSTN 36 will then drop the call to the second 
MTSO 33. 
Following task 98, a task 104 stores a data element 107 which describes the 
identity code obtained from cellular radiotelephone 20, hereinafter 
referred to as identity code 107. Identity code 107 is stored in 
association with substitute MIN 100 in an internal routing table 106, 
managed by primary MTSO 33. In the preferred embodiments, identity code 
107 includes at least a MIN and ESN for cellular radiotelephone 20. At any 
instant, internal routing table 106 may store any number of such 
associations for any number of substitute MINs 100. 
Next a task 103 records a current time stamp 109 for determining an 
assignment duration of substitute MIN 100. Time stamp 109 is recorded in 
association with substitute MIN 100 in internal routing table 106. 
Following task 103, a task 108 notifies the partner MTSO of the substitute 
MIN 100 assignment and associated identity code 107 for cellular 
radiotelephone 20. This communication takes place so that a call-back may 
be routed concurrently by PSTN 36 through the A-side and B-side system 
MTSOs 33 to cellular radiotelephone 20. After task 108, program control 
returns to a task 110, shown in FIG. 5. 
With reference back to FIG. 5, a task 110 participates in setting up the 
emergency call through primary MTSO 33 to PSAP 55 (FIG. 1). Enabling the 
emergency call from cellular radiotelephone 20 to PSAP 55 occurs via PSTN 
36. The responding PSAP 55 is informed that the emergency call originates 
from substitute MIN 100 when tasks 92, 98, 104, 103, and 108 were 
performed (FIG. 6). The emergency call proceeds normally through 
termination of the call. 
In response to task 108 (FIG. 6), partner MTSO 33 performs partner MTSO 
sub-process 84 (FIG. 5). Within sub-process 84, a task 112 monitors 
assignment notifications for substitute MIN 100. Those skilled in the art 
will appreciate that task 112 occurs in conjunction with other MTSO 33 
functions (not shown) that are not relevant to the present invention. 
In conjunction with task 112, a query task 114 determines if assignment 
notification is received by partner MTSO 33 from primary MTSO 33. It is 
upon notification of the substitute MIN assignment, that MTSO 33 becomes 
recognized as a "partner" relative to an emergency call. The MTSO 33 that 
is a "partner" on one emergency call may be a "primary" on another 
emergency call. Assignment notification includes communication of identity 
code 107 and substitute MIN 100. If assignment notification is not 
received, program control loops to task 112 to monitor for assignment 
notifications. 
If an assignment notification is received, a task 116 stores identity code 
107 for cellular radiotelephone 20 from the assignment notification. 
Identity code 107 is stored in association with substitute MIN 100 in an 
internal routing table 117 managed by partner MTSO 33. Routing table 117 
is equivalent to routing table 106 (FIG. 6) for primary MTSO 33. 
Program control for primary and partner modes of operating MTSOs 33 
proceeds from tasks 110 and 116, respectively, to a task 118. Task 118 
monitors for a call-back directed to cellular radiotelephone 20 and is 
performed by both primary and partner MTSOs 33 in conjunction with other 
functions. 
Next a query task 120, and subsequent tasks, are performed by both primary 
and partner MTSOs 33. Query task 120 detects call-backs directed to 
cellular radiotelephone 20. When no return call is detected, program 
control loops back to task 118 and monitors for call-backs directed to 
cellular radiotelephone 20. 
In connection with tasks 118 and 120, a background timing process (not 
shown) monitors time stamp 109 (FIG. 6) and drops the association between 
substitute MIN 100 and identity code 107 when a timing window expires. 
After substitute MIN 100 is dropped, it is marked as available in list of 
substitute MINs 102 (FIG. 6) and can be assigned to a subsequent emergency 
call. 
At query task 120, if a call-back is detected, process 80 proceeds to a 
paging routine 126. Paging routine 126 is performed by both primary MTSO 
33 and partner MTSO 33. Paging routine 126 is performed substantially 
concurrently by primary and partner MTSOs 33 with respect to call-backs 
directed to substitute MINs. 
FIG. 7 shows a flow chart of a paging routine performed by both primary and 
partner MTSOs 33. With reference to paging routine 126, a query task 128 
determines if the detected call-back is directed to a substitute MIN 100. 
When the call-back is not directed to a substitute MIN 100, a task 129 
processes the call in a conventional manner, and program control returns 
to a task 140 in process 80 (FIG. 5) to allocate a voice channel. 
When the call-back is directed to a substitute MIN 100, a task 130 then 
extracts MIN 74 and ESN 76 from internal routing table 106 (FIG. 4), or 
internal routing table 117 (FIG. 3). 
After task 130, a task 132 transmits a message addressed to MIN 74 and 
saves ESN 76. The message, or page order, is issued by MTSO 33 and 
transmitted as mobile station control message 72 (FIG. 2) over forward 
control channel 58 (FIG. 2). The page order contains MIN 74 and, as 
discussed above, MIN 74 need not be a unique number. 
Upon transmission of a page order, a query task 134 then monitors for a 
paging order confirmation message 78 (FIG. 2). If no paging order 
confirmation messages 78 are detected, a task 144 initiates a setup 
failure process. Task 144 takes the appropriate action to inform PSAP 55 
(FIG. 1) of setup failure. Program control then returns to task 118 (FIG. 
3) to monitor for other call-backs directed to cellular radiotelephone 20. 
While primary and partner MTSOs 33 perform task 134, cellular 
radiotelephone 20 monitors busy/idle bits 66 (FIG. 2). When reverse 
control channel 60 is idle, cellular radiotelephone 20 performs a system 
access by sending data over reverse control channel 60. The sent data 
contain MIN 74, ESN 76, and paging order confirmation message 78. 
Accessing reverse control channel 60 when busy/idle bits 66 are idle 
reduces the likelihood of two or more cellular radiotelephones that have 
the same MIN 74 from responding to a page order at the same time. When 
busy/idle bits 66 are switched to busy outside a predetermined timing 
window, a cellular radiotelephone conventionally aborts any transmission 
attempt and waits a random time interval before attempting to access 
reverse control channel 60 again. In response to an access attempt, 
busy/idle bits 66 (FIG. 2) of forward control channel 58 are switched to 
busy in a conventional manner (i.e. within the predetermined timing 
window), indicating that corresponding reverse control channel 60 is busy. 
Reverse control channel 60 is released when the access attempt is 
complete. 
When task 134 detects a paging order confirmation message 78 from the 
access attempt, the data transmitted on reverse control channel 60 is 
processed in a task 136, to determine if the conveyed ESN 76 matches ESN 
76 extracted from internal routing table 106 or internal routing table 
117. 
In query task 138, which is performed after task 136, paging order 
confirmation message 78 is evaluated to determine if comparison task 136 
indicates a match with ESN 76. When transmitted ESN 76 matches the ESN 76 
extracted from identity code 107, program control returns to task 140 in 
process 80 (FIG. 5) to process the call. The call is processed before 
another cellular radiotelephone accesses the system, and all subsequent 
access attempts are ignored. Referring briefly to FIG. 5, task 140 
allocates a voice channel for cellular radiotelephone 20 (FIG. 1) and 
performs other conventional call set-up functions. If call set-up is 
successful, task 140 then performs conventional call management functions. 
When cellular radiotelephone 20 is answered in response to the call, the 
voice channel is then enabled (i.e. connected through to PSAP 55). This 
causes the call-back call between cellular radiotelephone 20 and PSAP 55 
to progress in a conventional manner until the call terminates. When 
call-back succeeds through one MTSO 33, PSTN 36 drops the call to the 
other MTSO 33. Eventually program control returns to task 86, discussed 
above. 
When transmitted ESN 76 does not match the ESN 76 extracted from identity 
code 107, a query task 142 (FIG. 7) determines if there are other paging 
order confirmation messages 78. Paging confirmation message 78, conveying 
MIN 74, may be received from cellular radiotelephone 20, which has 
initiated an access delay until busy/idle bits 66 are switched to idle. 
Additional paging order confirmation messages 78 may be the result of an 
emergency call originating from cellular radiotelephone 20 with a 
non-unique MIN 74 (i.e. inactive). In other words, multiple cellular 
radiotelephones may respond to the page order transmitted in task 132. 
When there is another paging order confirmation message 78, program control 
loops back to task 136 to compare ESNs 76. This process is repeated for a 
plurality of paging order confirmation messages 78 in response to the page 
order. Task 136 and subsequent tasks repeat until paging order 
confirmation message 78 conveys a matching ESN 76, or until there are no 
further paging order confirmation messages 78 conveying MIN 74. 
When there are no further page order confirmation messages and a matching 
ESN 76 was not found, task 144 initiates a setup failure process. 
In summary, the present invention provides an improved emergency call-back 
process. The call-back process allows a return call to be performed over 
both A-side and B-side cellular systems concurrently. This allows for a 
greater probability in making a successful connection between an emergency 
caller and PSAP in the event of a call-back. This is especially desirable 
in the situation where the emergency call originates from an unauthorized 
cellular radiotelephone or from an unauthorized roamer. This call-back 
process can be implemented by cellular carriers at fairly nominal cost 
since this process is software based and resides in MTSO intelligence. No 
changes to the general population of existing cellular radiotelephones are 
required. 
Although the preferred embodiments of the invention have been illustrated 
and described in detail, it will be readily apparent to those skilled in 
the art that various modifications may be made therein without departing 
from the spirit of the invention or from the scope of the appended claims.