The gist of what you want is, low speed data services like faxing and dialup dont work over VoIP because of the way audio is mucked around when its packaged up to be sent via IP. There is no where near the bandwidth of a straight-up analogue lines. Then there is jitter which screws with it even more.
There is a protocol called T.38. But for it to work properly, needs to be implemented end-to-end and support is just generally very varying across all the different voice networks.