Not a direct answer but a curious one.

While it is impossible to use legacy modems with cellular voice channels, it is, in fact, possible to design a specialized modem which will work over it (more or less by plugging into a handset jack, as OP desires). This is a useful feature, because:

1. Data coverage may not be available in the remote areas where data communications may still be required (to support ATMs and EFT terminals, to name the most common need).
2. Voice channel has highest communication priority and will keep operating even if data services are completely overflown (by a massive crowd or spike in usage).

IEEE article describing a possible approach:

http://ieeexplore.ieee.org/xpl/login.jsp?tp=&arnumber=4383352&url=http%3A%2F%2Fieeexplore.ieee.org%2Fxpls%2Fabs_all.jsp%3Farnumber%3D4383352

Not a direct answer but a curious one.

While it is impossible to use legacy modems with cellular voice channels, it is, in fact, possible to design a specialized modem which will work over it (more or less by plugging into a handset jack, as OP desires). This is a useful feature, because:

1. Data coverage may not be available in the remote areas where data communications may still be required (to support ATMs and EFT terminals, to name the most common need).
2. Voice channel has highest communication priority and will keep operating even if data services are completely overflown (by a massive crowd or spike in usage).

IEEE article describing a possible approach:

https://ieeexplore.ieee.org/document/4383352

Not a direct answer but a curious one.

While it is impossible to use legacy modems with cellular voice channels, it is, in fact, possible to design a specialized modem which will work over it (more or less by plugging into a handset jack, as OP desires). This is a useful feature, because:

1. Data coverage may not be available in the remote areas where data communications may still be required (to support ATMs and EFT terminals, to name the most common need).
2. Voice channel has highest communication priority and will keep operating even if data services are completely overflown (by a massive crowd or spike in usage).

IEEE article describing a possible approach:

http://ieeexplore.ieee.org/xpl/login.jsp?tp=&arnumber=4383352&url=http%3A%2F%2Fieeexplore.ieee.org%2Fxpls%2Fabs_all.jsp%3Farnumber%3D4383352

A company which managed to turn the above approach into a successful commercial product:

http://www.symstream.com/dov.html

Not a direct answer but a curious one.

While it is impossible to use legacy modems with cellular voice channels, it is, in fact, possible to design a specialized modem which will work over it (more or less by plugging into a handset jack, as OP desires). This is a useful feature, because:

1. Data coverage may not be available in the remote areas where data communications may still be required (to support ATMs and EFT terminals, to name the most common need).
2. Voice channel has highest communication priority and will keep operating even if data services are completely overflown (by a massive crowd or spike in usage).

IEEE article describing a possible approach:

http://ieeexplore.ieee.org/xpl/login.jsp?tp=&arnumber=4383352&url=http%3A%2F%2Fieeexplore.ieee.org%2Fxpls%2Fabs_all.jsp%3Farnumber%3D4383352