Last I checked(it's been 15+ years) voice calls are 56kbps over a 64k channel(the extra was for signalling I believe) and data is 64kbps. Modem calls count as voice.
Back in the day when telcos starting doing more voip internally on their networks and I was still working for an ISP we noticed that our users were getting unusually slow connections and a lot of dropped calls.
After a few calls to our rep at PacBell she got to the bottom of it... according to her and a manager she brought on the live they were "upgrading" they network to carry more voice traffic by using voip and compressing the audio to save $$$... sometimes their equipment didn't recognize the call being made as coming from a modem and so would compress the audio and screw with the connections.
Turns out that there was an easy way around it. The compression was only happening on local calls when dialing 7 digits, so we had our users dial 10 digits and their connections were fine again.
I have no idea if that little trick still works or not, though I imagine if they *are* still compressing the calls trying to save bandwidth that their detection of modem/fax signals has been fixed... or at least I would hope.
Anywho... seems I'm rambling a bit... guess I'll stop now.