The V in VOIP stands for Voice.

VOIP will not carry data modem tones. As far as I know, the ONLY 'voice channel' that can also carry significant (relatively speaking) data bandwidth is indeed POTS.

What makes POTS unique is that it is essentially an UNcompressed 64Kbit data channel with a CODEC at each end, which is why it can be used for so many things; human voice, Fax, analog data modem, 56Kbit digital 'modem', etc.

When the transition from the old analog POTS phone central offices (telephone switching equipment) was planned the specification for the digital rendition was to (mostly) maintain compatibility with all the disparate uses the old analog POTS lines could handle. Hence the relatively flexible current POTS system.

VOIP, cellular voice, and other digital voice oriented compression/transmission systems all work by hugely focussing on and optimizing for the compressing of human voice 'sound fragments'. That same optimization works against every other form of sound, including modem data tones and modulations.

As far as I know there is no equivilent to POTS in the wireless world. Adapters and such cannot get around the limitations that voice compression systems do not handle modulated modem data well. You can create a 'voice' connection end to end with your home POTS line, but that connection is expecting to carry voice, not data.