If something stops working, and something was changed at the same time, then yes, I'd suspect that change probably has something to do with it. It may just be something that is not set correctly.
I would do a test...call someone, (or have someone call you), that uses a standard, analogue desk phone. The older type that will send DTMF as log as the key is depressed. During the call, have them send some DTMF tones to you. Are the tones there at all, get cut off (very short burst), low level, distorted ? If they don't sound right, then get your new provider involved, hell, get them involved anyway since the change that they made seems to have caused the problem. If the DTMF doesn't seem to get through during your test, then at least you have something to prove the problem when they show up.