I have just installed a SPA525G, upgraded the firmware to 7.4.3, then had excatly the same problem. I have had to revert to factory settings twice. It seems, although the evidence is only circumstancial, that the problem occurs after trying to change the ringtone. After the last time factory reset I did not try to change the ringone, and so far (after a few hours) the phone is still working normally.
Using the web interface, I was cycling through the supplied ringtones to choose one. In the end, I settled for "office". When I called the phone from another number, the lights flashed, but there was no audible ring. When I picked up the phone, the call was connected but there was no voice in either direction. I did a factory reset, and then used the phone inteface to select the same ringtone. The same thing happened - no ring and no audio. I reset the phone again, and did not change the ringtone. So far, so good.
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