What
          kinds of third party SIP are you all having so much issue
          with?  I manage a lot of accounts using the big, hosted
          providers and plenty of the endpoints sit behind
          Xfinity/Comcast boxes without issue.

The
          dropping registrations just sound like timer and firewall
          configurations.  By rule, I try to always go bridged mode with
          Comcast provided boxes, but even when not I can't recall
          having an issue like this except via the normal things like
          ALG being enabled or some type of security inspections causing
          trouble.  And TLS is the way 100%