Starting roughly on 9 May 2013, I was having trouble sometimes (Mostly calls from T-Mobile cell phones). Our home phone would ring, but would not pick up (they would still hear ringing), or it would pick up, but we could not hear the caller. Sometimes it would go to voicemail/
I had to ENABLE SIP ALG on my Linksys DIR-655 (HW: B1, FW: 2.10NA) under "Advanced", then "Firewall Settings".
Then everything started working again!
Callcentric noted that this may be related to the Colorado DID provider, not T-Mobile specifically, as they have had similar reports.
My experience, YMMV, "The opinions expressed herein are not my own", etc.
[UPDATE:] Perhaps the fix was just the reboot of the router that was required for the SIP ALG change. It seems to work intermittently, regardless of the SIP ALG setting.