sooooooooo frustrating. I'm having same issue. Just signed up for Triple Play service a few days ago and having this same exact issue with my iPhone 6 plus. Frustrating that this known issue hasn't been resolved and when I call into 1-800-COMCAST and technical support acts like this is the first time they heard it.
Whether it is the fault of Apple, AT&T, Comcast, or some combination thereof, if its a major issue, the FCC regulates the number portability process. What Device dials a PSTN Connected Number should have no bearing on whether a call completes (That whole Common Carrier, thing...)
If numbers are not porting properly, attempt resolution with your provider, in this case Comcast, and if not satisfactory, that is why the FCC has a Complaint Process
So in theory, is this related to VoLTE and routing between AT&T and Comcast? One way for end users to check: Disable LTE Voice (ie Setting -> Cellular -> Enable LTE -> Data Only instead of Voice & Data) on the iPhones that aren't routing the calls, and see if that corrects the issue
Would make sense why a newer iPhone would have problem, and if AT&T disables LTE Voice access for that device from their side, it would complete the calls, too...
I don't know if that is whats going on, but its a theory that fits the given facts