A follow up:
Tested the optipoint 420 with V7 R1.4.0 and all seemed fine. Updated phone firmware to most current V7 R5.6.0 and did more testing. All basic functions seemed to work fine. Make calls, receive calls, transfer, hold, conference, even multiline (even though there was no led status on optipoint using soft phone as main line). Phone stayed registered. Only issue I did notice was Music on Hold does not work when the Siemens Optipoint places a call on hold. --- Supposedly the hold music on these should come from a file stored in the phone set, but there is notation in Siemens wiki that MOH is not supported in SIP version of the optipoint. So, seems it functions with some restrictions using the 3CX phone system V8.0.10824.716.
There is a listing in the release notes for optipoint earlier firmware version for "phone set occasionally unregisters". Possibly, answer to the original problem of registration issues... Give Update to current firmware a try.
I like this phone for some of the keyset type features, self-labeling, etc. -- Could be viable if moh issue and lack of template resolved, but still there are probably more cost effective solutions with better compatibility.
Yon l