I don't know if I'm blaming Callcentric unjustly, but here's the issue:
I just switched my sp1 to Callcentric free DID from ipkall/localphone since lp was dropping calls. Now CC seems to be working fine (not dropping calls), but today when I called a friend who has Cricket cell service (cdma and suspected voip for cell calls), after 3-4 minutes into the call, his voice went extremely tinny, almost like a whisper talking through a voice scrambler. He said he could hear me fine. I called back and the same thing. Called a 3rd time same thing (after a few minutes). Hung up and called someone else on a landline, no problems. Hung up and called back the original friend, but using GV this time, no problems. After about 30 mins, called back the same guy on CC again... and after a good start, 3.5 minutes in, the voice got crappy again. So, in this story, it happens when calling with Callcentric, and the other party on Cricket.
Any ideas?
If you can repeat it, then you might want to open a ticket with Callcentric. That's the best way to figure it out.