poor quality of incoming calls!!

Hi
I am using a Linksys spa3102 which is plugged directly into my cable modem.
I am using it as a router. I did get a PAP2T and had it
behind my router but because I was having problems, I installed the other
unit to rule out the router. Making outbound calls works great and quality
is fine. But when I receive an inbound call, the callers voice is hard to
make out because there is a lot of background noise/static and their voice
is garbled (not jitter). Also some inbound calls, I answer but there is no
one on the other end – either I can’t hear them or they hang up – not sure.
I have also disconnected all the other devices on the network so that I know
that the only use of the internet is for VOIP – but no improvement.

What do you think?Any help please..

Thanks