Fax and Modem support

Maybe because the voice codec you're using is G.711, or even G.726 32kpbs, which are adequate for transfer of fax and modem traffic. Or, it's possible that your VoIP provider "upspeeds" fax and modem traffic (essentially allocating more bandwidth to these applications when their use is detected).

Simply *enabling* "Fax and Modem Support" should not degrade anything. of course, actually *using* the service will, because it will rob bandwidth from your voice app.

James