I haven't tried it, but I believe the voice codecs used for the analogue to digital conversion often recognise fax/modem signals and transmit them as raw data to conserve bandwidth. The speed may be limited to the speed of the voice channel, which is something like 9.6 KBPS.
best way to find out is to try it, as others have said it almost certainly depends on the implementation. If it doesn't work you may be able to get it to by forcing a different codec at the voip box.
If you need to find a tradesperson to get your job done, please try our local search below,
or if you are doing it yourself you can find suppliers local to you.
Select the supplier or trade you require, enter your location to begin your search.
Please select a service and enter a location to continue...
Are you a trade or supplier? You can create your listing free at DIYnot Local