UPDATE: IT WORKS!
I did a clean install of v2.6 and selectively imported sections from the prior config.; specifically the OpenVPN, System, FW aliases (NOT rules), DHCP and DNS forwarder services. I did add an 'allow any-any' rule to the OpenVPN interface, but the WAN and LAN interfaces were left at default (basically empty).
I did add DHCP options 066 and 160 to specify a provisioning server rather than manually entering it on the phone. A factory reset of the phone did the expected; downloaded a config. and registered with the PBX at the remote site. It can make and receive calls normally.
I can't honestly say what the root cause was so it will just have to remain a mystery.