Line interface unit for caller-controlled receipt and delivery of voice messages

A line interface unit for controlling receipt and delivery of voice messages over a telephone system having a voice message facility connectable to at least one calling station and a called station via a central office. The interface unit comprises a first switch connected to the calling station in the tip and ring lines and a second switch connected to the central office in the tip and ring lines, the switches having first and second positions and wherein said signals on the tip and ring lines are connected directly between the calling station and the central office when each of the switches is in the first position. In operation, a control circuit of the line interface unit responds to detection of a busy/no answer condition at the called station for switching the first switch from its first to second position to connect a speech circuit to the calling station for issuing a prompt. After the prompt is issued, the control circuit responds to receipt of a DTMF signal or additional coinage from the calling station for switching the second switch from first to second position to connect a speed-dial circuit to the central office for dialing the voice message facility. The control unit then responds to receipt of a DTMF signal from the voice message facility for switching the first and second switches from their second positions back to their first positions to connect the calling station to the voice message facility.

TECHNICAL FIELD 
The present invention relates to voice messaging methods and systems and 
more particularly to a line interface unit for controlling receipt and 
delivery of voice messages over a telephone system having a voice message 
facility connectible to a calling station and a called station via a 
central office. 
BACKGROUND OF THE INVENTION 
It is known in the art to operate a multiple paystation telephone 
installation from a central office. In such systems, the central office 
typically performs the coin collect, return and verification functions 
necessary to provide both local and long distance services. Multiple 
paystation installations of the type described above have recently been 
enhanced through the use of so called "smart" paystations. Such 
paystations typically include their own microprocessor and associated 
memory for supporting various computer programs. These programs provide 
several desirable operating features, e.g., loop start dialtone and "least 
cost routing" for long distance telephone calls. Although "smart" 
paystations are advantageous, such devices have yet to be successfully 
integrated into voice messaging systems. Paystation operators have 
therefore been unable to offer message storage/forwarding to users of 
their paystations. 
It would therefore be desirable to provide a line interface unit for use in 
controlling receipt and delivery of voice messages in a telephone system 
which overcomes these and other problems of the prior art and which 
facilitates the providing of certain call default and message prompting 
operations in a standalone unit. 
BRIEF SUMMARY OF THE INVENTION 
A line interface unit is described for controlling receipt and delivery of 
voice messages over a telephone system having a voice message facility 
connectible to at least one (pay or non-pay) calling station and a called 
station via a central office, the calling station having tip and ring 
lines connected to the central office. The interface unit comprises a 
first switch connected to the calling station in the tip and ring lines 
and a second switch connected to the central office in the tip and ring 
lines, each of the switches having first and second positions and wherein 
signals on the tip and ring lines are connected directly between the 
calling station and the central office when each of the switches are in 
the first position. The interface unit also includes a speed-dial circuit 
for dialing the voice message facility, a speech circuit for issuing a 
predetermined prompt to the caller at the calling station, a line monitor 
circuit for monitoring the tip and ring lines, and a microprocessor-based 
control circuit connected to the line monitor circuit and the first and 
second switches. The line monitor includes first and second detector 
circuits, the first detector circuit for detecting a busy/no answer 
condition at the called station upon call initiation to the called 
station. The second detector circuit preferably detects receipt of a 
standard dual tone multifreguency (DTMF) signal from the calling station 
and receipt of a standard or non standard DTMF signal from the voice 
message facility. 
In operation, the control circuit responds to a busy/no answer condition at 
the called station for switching the first switch from its first position 
to its second position to connect the speech circuit to the calling 
station for issuing the prompt. After the prompt is issued, the control 
circuit responds to receipt of the DTMF signal from the calling station or 
the input of additional coinage for switching the second switch from its 
first position to its second position. This switching connects the speed 
dial circuit to the central office for dialing the voice message facility. 
The control circuit then responds to receipt of the DTMF signal from the 
voice message facility for switching the first and second switches from 
their respective second positions back to their respective first positions 
to thereby connect the calling station to the voice message facility. 
In an alternate embodiment, the calling station is a paystation. In this 
embodiment, the line interface unit includes a third switch connected 
between the ring line and ground for generating a ground start after the 
voice message facility has been dialed by the speed dial circuit. This 
operation enables the interface unit to complete a connection to the voice 
message facility through the central office without the caller having to 
deposit additional coinage. After the ground start, the control circuit is 
responsive to receipt of the DTMF signal from the voice message facility 
for switching the first and second switches from their respective second 
positions back to their respective first positions to thereby connect the 
calling paystation to the voice message facility. If the caller does not 
desire to leave a voice message, however, the line interface unit causes 
the central office to generate a "coin return" signal to return the 
caller's coinage. 
In yet another embodiment, the line interface unit provides a so called 
"voice over" operation wherein the caller at the calling station is 
allowed to continue to monitor the call progress (i.e., the ringing on the 
line) while being prompted to leave a message. In this embodiment, the 
control circuit is responsive to predetermined condition at the called 
station for switching the first and second switches from their respective 
first positions to their respective second positions. This operation 
connects the speech circuit to the calling station for issuing the prompt 
while simultaneously enabling the caller at the calling station to 
continue to monitor the progress of the call. After the prompt is issued, 
the control circuit is responsive to receipt of a DTMF signal from the 
calling station for switching the second switch from its second position 
to its first position and then back to its second position. This operation 
reorders dialtone to the interface unit and then connects the speed dial 
circuit to the central office for dialing the voice message facility. 
After dialing the voice message facility, the control circuit is 
responsive to receipt of the DTMF signal from the voice message facility 
for switching the first and second switches from their respective second 
positions back to their respective first positions and thereby connect the 
calling station to the voice message facility.

DETAILED DESCRIPTION 
FIG. 1 is a block digram of a multiple user station telephone system 10 in 
which a voice message storage and forwarding method has been implemented 
according to the teachings of U.S. Pat. No. 4,766,604, assigned to the 
assignee of the present application. System 10 includes a plurality of 
user stations 12a-12n connected to a conventional or private owned central 
office 14. Each of the user stations 12a-12n is preferably a "pre pay" 
paystation, such as a Model 1200-2 manufactured by Elcotel, Inc., and thus 
includes a suitable microprocessor and associated memory device (e.g., a 
programmable read only memory or "PROM") for storing computer programs 
which control the operation of the paystation. Each paystation also 
includes a conventional handset 15 as well as other standard components 
(not shown) normally associated with the coin collection process. 
To initiate a call from one of the paystations 12a-12n, a caller removes 
the handset 15 and enters a called number. This number identifies a called 
station 16 which may be either a local or a long distance station. As also 
seen in FIG. 1, the multiple paystation system 10 includes a voice message 
facility 18 connected to the central office 14 for recording voice 
messages from the user stations 12a-12n. Voice message facility 18 has a 
plurality of access ports 19 from which recorded voice messages are 
output. The system 10 may also include a call accounting unit 20 connected 
to the voice message facility 18 for the purposes to be described. 
Multiple user station systems such as shown in FIG. 1 as well as other 
telephone systems having non pay calling stations have been enhanced by 
the method for receiving the delivering voice messages described in U.S. 
Pat. No. 4,766,604. The method exploits the inability of a calling station 
12a-12n to complete a call to the called station 16 to automatically cause 
the called station to default to the voice message facility 18. If the 
caller desires to leave a voice message to be delivered to the called 
station 16, the caller is thereafter prompted to store the voice message. 
The voice message facility 18 then initiates an outdial routine which 
makes a predetermined number of attempts to forward the voice message to 
the called station. Each of these attempts are typically made at a 
predetermined time interval programmed by the system operator. 
Referring now to FIG. 2, a flowchart diagram is shown detailing a call 
routine 100 of the method for receiving and delivering voice messages in a 
paystation embodiment as described in U.S. Pat. No. 4,766,604. The call 
routine 100 begins at step 102 when the calling station handset 15 goes 
off hook. As described in that application, each calling station 12a-12n 
is preferably a "pre pay" paystation which requires receipt of appropriate 
coinage before a call can be placed. Accordingly, at step 104, an inquiry 
is made to determine whether a coin has been received. If not, the method 
continues at step 105 to request that the caller deposit the appropriate 
coinage for the call. Upon receipt of the coinage, the method continues at 
step 106 to deliver the coin to a conventional coin escrow chamber of the 
calling station. 
At step 108, the method receives and stores the called number identifying 
the called station 16. At step 110, the called number is transmitted over 
the trunk and the calling station waits for an answer detection (i.e., the 
called station goes off-hook). The method continues at step 112, where an 
inquiry is made to determine whether the call can be completed. In 
particular, step 112 preferably determines whether answer detection occurs 
within a predetermined number of rings or whether the called station is 
busy. If the call can be completed, the method continues at step 114 to 
release the coin from the coin escrow chamber into the coin vault of the 
calling station. At step 116, the call is completed in a normal fashion. 
If the result of the inquiry at step 112 indicates that answer detection 
has not occurred (i.e., the called station remains on hook) within the 
predetermined number of rings following call initiation or that the called 
station is busy, the method initiates a "call default" routine 118 for 
determining whether the caller at the calling station desires to leave a 
voice message to be delivered to the called station. 
Referring now to FIG. 3, a detailed flowchart diagram is shown of the call 
default routine 118 of FIG. 2. The routine begins at step 120 wherein the 
calling station issues a prompt to the caller such as "Please Wait" or 
"One Moment Please". A bilingual prompt may also be issued at step 120 if 
desired. At step 122, the handset of the calling station is disabled. The 
call default routine 118 continues at step 124 wherein the original call 
is disconnected from the trunk and a "speed dial" is initiated to the 
voice message facility 18. The muting of the handset at step 122 is 
advantageous because step 124 disconnects the original call from the 
trunk. If the handset were not muted the caller would otherwise hear a 
dialtone and the voice message facility 18 could be accessed via a "black 
box" or other illegal means. 
The call default routine 118 continuous at step 126 with the handset of the 
calling station reactivated upon receipt of a predetermined signal, e.g., 
a DTMF (dual tone multifreguency) signal identifying the "#" key, from the 
voice message facility. At step 128, the voice message center issues a 
prompt to the caller and waits for an appropriate response. Although not 
meant to be limiting, the following prompt is issued at step 128: 
"This is a voice message center. Your party is not available at this time. 
If you would like to leave a 20 second message at no additional charge, 
press the " #" key and I will attempt to deliver your message every 15 
minutes for the next two hours. If you do not wish to leave a message, 
hang up, and I will return your money." 
The call default routine 118 continues at step 130 to determine whether the 
caller desires to leave a voice message for the called station. In 
operation, step 130 waits for reception of a DTMF signal identifying the 
"#" key within a set period of time, e.g., 15-20 seconds. If the timer at 
step 130 times out (indicating that the caller does not wish to leave a 
voice message) the call default routine 118 continues at step 132 with the 
voice message facility 18 sending a "disable" signal to the calling 
station. At step 134 in the paystation embodiment, the coin located in the 
coin escrow chamber is released to a coin return chute of the calling 
station and the call is terminated. If the result of the inquiry 130 is 
positive, the method continues by calling a "call message" routine 136 for 
storing the voice message to be delivered to the called station. 
Referring now to FIG. 4, a detailed flowchart diagram is shown of the call 
message routine 136 of FIG. 3. The routine begins at step 138 wherein, in 
the paystation embodiment, the calling station releases the coin from the 
coin escrow chamber into the coin vault of the calling station. At step 
140, the calling station transmits the called number to the voice message 
facility 18. The call message routine 136 continues at step 142 wherein 
the calling station transmits a preferably four digit security code "YXXX" 
to ensure that the calling station is entitled to transmit messages to the 
voice message facility. Preferably, the first digit "Y" of the security 
code is a DTMF combination of a dialtone and the "#" key. The remaining 
three digits "XXX" of the security code are preferably a number which 
uniquely identifies the calling station. 
At step 144, the received security code is evaluated to determine whether 
the calling station is entitled to transmit messages to the voice message 
facility 18. If the calling station is invalid, the call message routine 
136 issues a disconnect signal to the calling station at step 146, thereby 
disconnecting the calling station from the voice message facility 18. If 
the result of the inquiry 144 indicates that the calling station is valid, 
the voice message facility 18 issues a prompt at step 148 to request that 
the caller begin the message. For example, the prompt at step 148 may 
state: 
"Please state your name and your 20 second message now." 
At step 150, an inquiry is made to determine whether the caller has begun 
recording the message within a predetermined time, e.g., 5 seconds. If the 
caller has not begun recording the message within the predetermined time, 
the call message routine 136 issues a salutation and then generates a 
disconnect signal at step 146. 
If the result of the inquiry at step 150 is positive, the voice message 
facility 18 records the spoken message at step 152. At step 154, the call 
message routine 136 time stamps the message and stores the message and the 
called number transmitted at step 140 in a call.queue. As used herein, the 
phrase "time stamps" mean that the message and the called number are 
placed in the call queue (i.e., a suitable memory device of the voice 
message center) at an address uniquely associated with the time that the 
caller began recording the message. At step 156, an index "N" for the 
message is initialized. The index "N" represents a number of outdial 
attempts to be performed by the voice message center 18 as will be 
described. The call message routine 136 terminates at step 158 by calling 
an "outdial to completion" routine which is used to attempt delivery of 
the stored voice message to the called station. 
The outdial-to completion routine attempts to forward each stored message 
at a predetermined time interval (and multiples thereof) from the time the 
message is stored in the call queue. The message may be forwarded 
redundantly to insure delivery to the calling party. Alternatively, the 
message is forwarded once after a short time delay (e.g., 2-3 seconds) to 
insure that the calling party is attentive prior to delivery. 
While the invention described in U.S. Pat. No. 4,766,604 has proven 
satisfactory, it has been found desirable to provide certain enhancements 
and improvements to the voice messaging method described therein. 
Referring now to FIG. 5, which is a modification of the arrangement of 
FIG. 1, a line interface unit 200 is shown for providing enhanced control 
of receipt and delivery of voice messages over a telephone system having a 
voice message facility 18 connectible to at least one calling station 12 
and a called station 16 via a central office 14. As will be described, the 
line interface unit 200 advantageously provides certain call default and 
message prompting operations in a standalone unit both externally to the 
calling station 12 and the voice message facility 18. The interface unit 
may be located adjacent to the calling station or on the input side of the 
central office switching equipment. 
As seen in FIG. 5, the line interface unit 200 comprises a plurality of 
functional modules including first, second and third switches 202, 204 and 
206, respectively, DTMF detector circuits 208a and 208b, a speech circuit 
210, a speed dial circuit 212, a coin detect circuit 214, a call progress 
detector circuit 216, a control circuit 218, a calling station monitor 
circuit 224 and a central office monitor circuit 226. The control circuit 
218 is preferably a microprocessor based controller having suitable 
control programs for effecting the operations described below. The control 
circuit 218 is connected to control the first, second and third switches 
202, 204 and 26, as well as the speech circuit 210 and the speed dial 
circuit 212. The speech circuit 210, the speed-dial circuit 212 and/or the 
control circuit 218 may be shared by a plurality of line interface units 
each having the disclosed switching and monitoring components. 
In the preferred embodiment of the invention, each of the first and second 
switches are a double-pole, double throw switch located in the tip and 
ring lines 220 and 222 between the calling station 12 and the central 
office 14. In particular, the first switch 202 is connected to the calling 
station 12 in the tip and ring lines 220 and 222 and the second switch 204 
is connected to the central office 14 in the tip and ring lines. Each of 
the switches 202 and 204 have first and second positions such that signals 
on the tip and ring lines 220 and 222 are connected directly between the 
calling station 12 and the central office 14 when each of these switches 
are in the first position. The unit is thus "idle" when the switch 
contacts of the first and second switches 202 and 204 are in their first 
position. However, the line interface unit 200 communicates directly with 
the calling station 12 when the first switch 202 is switched from its 
first position to its second position. Likewise, the line interface unit 
communicates directly with the central office 14 when the second switch 
204 is switched from its first position to its second position. The unit 
200 thus communicates with both the calling station 12 and the central 
office 14 when the switch contacts of switches 202 and 204 are activated 
to the second positions. When both switches are in their second position, 
a small amount of crosstalk between the switches 202 and 204 exists and is 
advantageously used by the interface unit 200 for the purposes described 
below. Because of the placement of the switches 202 and 204 in the tip and 
ring lines 220 and 222, it should be appreciated that switching of the 
first switch 202 from its first position to its second position causes the 
handset of the calling station to be disconnected from the central office 
14. Likewise, switching of the second switch 204 between its first and 
second positions automatically disconnects the calling station from the 
central office 14 and reorders dialtone to the line interface unit 200. 
As noted above, the line interface unit 200 also includes the speech 
circuit 210 for issuing one or more predetermined (English or bilingual) 
prompts to the caller at the calling station 12. For example, the speech 
circuit is controlled by the control circuit 218 to issue a prompt to 
determine whether the caller at the calling station desires to leave a 
voice message for a called station. If the caller desires to leave a voice 
message, the control circuit 218 operates to control the speed dial 
circuit 212 for speed-dialing the voice message facility 18. The control 
of this operation is effected through sensing of various signals by the 
line monitor circuits 224 and 226. 
In particular, the calling station monitor 224 is connected across the tip 
and ring lines 220 and 222 adjacent the calling station side of the unit 
200. Likewise, the central office monitor circuit 226 is connected across 
the tip and ring lines adjacent to the central office side of the 
interface unit 200. The calling station monitor 224 monitors the tip and 
ring lines for coin detect signals (which are processed by the coin detect 
circuit 214) or receipt of a predetermined DTMF signal (e.g., a "#" key) 
from the calling station (which is processed by the DTMF detector circuit 
208a). The central office station monitor 226 monitors the tip and ring 
lines for detecting a predetermined condition at the called station (e.g., 
a busy/no answer condition) upon call initiation to the called station. 
Central office monitor circuit 226 also monitors the tip and ring lines 
for detecting receipt of standard or non standard DTMF signaling which is 
then processed by the DTMF detected circuit 208b. The control circuit 218 
includes a switch 219 for selectively switching the outputs of the DTMF 
detector circuits 208a and 208b to the control circuit 218. 
Although not meant to be limiting, preferably the DTMF detector circuit 
208a senses receipt (over the calling station monitor 224) of a standard 
DTMF signal indicating that the caller at the calling station desires to 
leave a voice message for the called station. Alternatively, the caller 
can deposit additional coinage or pull a credit card through a cardreader 
to indicate his/her intention to record a voice message at his or her 
expense. The calling party thus controls the message storage (by entering 
a predetermined code following the message prompt) and is responsible for 
paying for the message forwarding attempts. Call accounting unit 20 as 
shown in FIG. 1 advantageously monitors and records all such message 
storage and forwarding activities for each calling station to enable the 
system operator to directly bill the calling station for the service. The 
DTMF detector circuit 208b senses receipt (over the central office monitor 
226) of a non standard DTMF signal from the voice message facility 
following the speed-dialing thereto as will be described in more detail 
below. 
The third switch 206 of the line interface unit is connected between the 
ring line 222 and ground for the purpose of generating a ground start 
after the speed dial circuit 212 has speed dialed the voice message 
facility. This ground start is required to enable the interface unit 200 
to complete a connection to the voice message facility 18 through the 
central office without the caller having to deposit additional coinage in 
the paystation. As also seen in FIG. 5, the line interface unit 200 
includes the coin detect circuit 214 for detecting receipt of coinage in 
the paystation. Coin detect circuit 214 is preferably coupled to the 
control circuit 218 to provide a running total of all coins deposited in 
the calling station over a predetermined time. Moreover, the control 
circuit 218 can be remotely reprogrammed (e.g., to change the prompt, the 
language of the prompt, the calling station identification or the number 
of unanswered rings at the called station which will initiate a call 
default) through DTMF signaling from a host or handset. To effect 
reprogram ming, the second switch 204 is switched to its second position 
and the parameter information is down loaded to the microprocessor. 
In operation, the control circuit 218 responds to a busy/no answer 
condition at the called station (following call initiation thereto) for 
switching the first switch 202 from its first position to its second 
position. This operation disconnects the calling station handset from the 
central office and connects the speech circuit 210 to the calling station 
12 for issuing the prompt to determine if the caller desires to leave a 
voice message. After the prompt is issued, the calling station monitor 
circuit 224 waits for receipt of the DTMF signal from the calling station 
(or, alternatively, waits for receipt of additional coinage). If no DTMF 
signal is received (i.e., if the caller does not desire to leave a voice 
message), the unit 200 remains idle. However, upon detection of the DTMF 
signal by the DTMF detector circuit 208a (or the detection of additional 
coinage by coin detector 214 as the case may be), the control circuit 218 
responds by switching the second switch 204 from its first position to its 
second position. This switching reorders dialtone and connects the speed 
dial circuit 212 to the central office 14 for dialing the voice message 
facility 18. The central office monitor circuit 22 then waits for receipt 
of the DTMF signal from the voice message facility. The control circuit 
218 then responds to detection of the DTMF signal by the DTMF detector 
circuit 208b for effecting a "handshake" to the voice message facility and 
then switching the first and second switches 202 and 204 from their 
respective second positions back to their respective first positions to 
thereby connect the calling station 12 to the voice message facility 18. 
As noted above, when the calling station 12 is a paystation, the line 
interface unit 200 must present a ground start to the central office 14 in 
order to dial the voice message facility 18 without additional coinage. To 
this end, the control circuit 218 operates to activate the third switch 
206 connected between the ring line 222 and ground for generating a ground 
start after the voice message facility 18 has been dialed by the speed 
dial circuit 212. This ground start is typically performed immediately 
after the last digit of the voice message facility is dialed by the 
speed-dial circuit and enables the interface unit to complete a connection 
to the message facility without the caller having to deposit additional 
coinage. After the ground start, the control circuit 218 is responsive to 
receipt of the preferably non standard DTMF signal from the voice message 
facility 18 for switching the first and second switches from their 
respective second positions back to their respective first positions. This 
operation connects the calling paystation to the voice message facility. 
The use of non standard DTMF signaling increases security of the 
transmissions between the calling station and the voice message facility. 
In the paystation embodiment, it should be appreciated that switching of 
the first switch 202 to the second position (e.g., in response to a 
busy/no answer condition at the called station) causes the central office 
14 to issue a 130V "coin return" signal to the paystation. This signal, 
however, cannot reach the calling station because the first switch is in 
the second position. Accordingly, when the caller at the calling station 
does not desire to leave a voice message for the called station, the line 
interface unit 200 must cause the central office to issue another "coin 
return" signal. Otherwise, the caller will not obtain his/her coinage back 
from the paystation. This "coin return" operation is achieved according to 
the invention when the caller goes "on hook" (indicating that the caller 
does not desire to leave a voice message). In particular, at that time the 
control circuit 218 switches the second switch 204 to its second position 
to reorder dialtone from the central office 14. Thereafter, the first 
switch 202 is switched back to its first position. After the central 
office monitor circuit 226 senses dialtone, the control circuit 218 
switches the second switch back to its first position. To the central 
office 14 this operation emulates the calling station going off hook and 
then on hook. The central office 14 thus again issues a "coin return" 
signal; however, this signal is not passed through the "idle" line 
interface unit to the calling station. The caller s original coinage is 
then returned. 
In yet another embodiment, the line interface unit provides a so-called 
"voice over" operation wherein the caller at the calling station 12 is 
allowed to continue to monitor the call progress (i.e., the ringing on the 
line) while being prompted to leave a message. In this embodiment, the 
control circuit 218 is responsive to a predetermined condition at the 
called station (e.g., three (3) rings with no answer) for switching the 
first and second switches from their respective first positions to their 
respective second positions. This operation connects he speech circuit 210 
to the calling station 12 for issuing the prompt while simultaneously 
enabling the caller at the calling station to continue to monitor the 
progress of the call. This operation is effected by the crosstalk between 
the first and second switch transformers as best seen in FIG. 6, which is 
a detailed schematic diagram of the switches 202 and 204. In particular, 
first switch 202 includes a control relay 236 controlled by the control 
circuit 218 for switching the switch between the sets 240a and 240b of 
switch contacts. When the switch is in the second position (across 
contacts 240b), the speech circuit 210 is coupled to the calling station 
via the line transformer 242. Likewise, when the second switch is in its 
second position (across contacts 240 d), the speed-dial circuit 212 is 
coupled to the central office and thus the voice message facility via the 
line transformer 244. Crosstalk between the line transformers 242 and 244 
enables the voice-over operation to be effected. 
After the prompt is issued during the voice-over operation, the control 
circuit 218 is responsive to receipt of a DTMF signal from the calling 
station 12 for switching the second switch from its second position to its 
first position and then back to its second position. This operation 
reorders dialtone to the interface unit 200 and then connects the speed 
dial circuit 212 to the central office for dialing the voice message 
facility 18. After dialing the voice message facility, the control circuit 
is responsive to receipt of the DTMF signal from the voice message 
facility for switching the first and second switches from their respective 
second positions back to their respective first positions and thereby 
connect the calling station to the voice message facility. 
Before the first and second switches are returned to their first position 
to connect the calling station to the voice message facility, the line 
interface unit 200 performs a "handshake" with the voice message facility 
18. In particular, the control circuit 218 transmits a predetermined data 
string to the voice message facility 18. Preferably, this data string has 
the following format--"#", "#", (10 digit number of calling station, "#", 
(1-digit activity code), "#", (called station number), "#"--. The data 
string may also include a credit card number. The "#" sign delimits the 
fields of the data string. The activity code informs the voice message 
facility 18 or other host that an alternative prompt has been made (e.g., 
a bilingual prompt). One of more of the "#" signs may alternatively be 
non-standard DTMF signaling for increased security. When the voice message 
facility receives the data string, it transmits an "acknowledge" signal 
(e.g., an "*") back to the unit or requests retransmission if the data has 
not been received. Upon receipt of the acknowledge signal, the control 
circuit 218 switches the first and second switches back to their 
respective first positions. 
While in the preferred embodiment the line interface unit 200 is adapted 
for use with a single calling station, a plurality of calling stations may 
be selectively switched to a single interface unit. Moreover, although not 
discussed in detail, it should be appreciated that other forms of 
signaling may be used in the invention. For example, the control circuit 
218 may include appropriate circuitry for processing FSK signaling if 
desired. Further, the control circuit 218 of the line interface unit 200 
may alternatively include appropriate software to perform automatic number 
identification ("ANI") following the caller's affirmative decision to 
leave a voice message. This ANI function is advantageous when the calling 
station and the called station are intra-lata. 
The following is a source code listing of a preferred control program for 
controlling the control circuit microprocessor to carry out the functions 
described above: 
##SPC1## 
Although the invention has been described and illustrated in detail, it 
should be appreciated that the same is by way of illustration and example 
only. The spirit and scope of the present invention are to be limited only 
by the terms of the appended claims.