Hi Somia,
My guess is that both, the voice modem and the telephone are not configured to work in the same carrier or have different configurations...
You can see in this page what kind of configuration can v.23 have, if you read the v.23 specifications, it is standar for a switched telephone netowrk and can work for both, 600 and 1200 baud.
https://www.arcelect.com/vspecifi.htm
Here is a link for a complete guide of AT commands, download the pdf it is really good as i can see.
https://www.arcelect.com/AT_Basic_Commands.htm
I haven't really done it myself, a couple of years ago i connected two socket modems using PICS, i had some work over the hyperterminal till i got the sequence right for the conecction....
You got to set all the parameters manually, like the role of the modem, carrier, responce times, bit rate, how many data bits are sent, parity and all that...
Hope this gives you a clue.