Followup - we just turned off STUN as we no longer needed it with this setup, and we can't seem to make the phone lockup.  It seems highly likely to revolve around STUN server usage.  (We also have NAT turned on FYI)

We have a SPA504G with the same issue and can reproduce the results...All 4 lines are connected to Teliax (SIP provider) with different registrations.Line 1 on Phone is set in Teliax as "Extension 101"Line 2 on Phone is set in Teliax as "Extension 10...