Anyway I have just tested it. I connected a smart phone from it's hands free port to the PC mic and phones. then I called the smart phone from a land line at home.
It could not even send 110 baud. It started sending them ok but after 1-3 seconds they were suppressed. Continuously changing the AF volume to 0% and then back to a percentage tricked the codec somehow.
Sending pulses of a single tone passed ok.
Also, sending a dual tone passed ok.
But not FSK at 110, 300 or 1200baud, they all seemed to have this problem I mentioned.
Also when sending the above and tried to also send FSK at the same time, again everything was ceased at the channel and nothing heard at the other end, even my sound card was output tones.
I have also tried SSTV and the result was that some tines refused to be transmitted.
Now, I do not know if this is due to the mobile phone codecs or the landline phone codecs, as I have not tried it with two mobile phones yet.
I may have a more detailed analysis on it, maybe note which tones pass ok and which not, or if it is not a tone problem, identify why the codecs cause silence on the channel.