In true BT fashion for consistency, you can oddly pair handsets to the hub without DV but not DV adapters!!
I'll let you know. In fact I have a spare BNIB, BT digital phone which has never been paired. I'll try that too. If that doesn't work, then the BT phone is miss-named as DECT, as it so 100% proprietary to BT. Interesting to note the EE store has them clearly labelled as for BT voice service only. That wasn't clear a couple of years back and for such a fundamental piece of hardware is a backward step. Great if BT want to advance development, but keep compatibility with long standing standards. Further contributions to the e-waste mountain are not good for the planet.
So here is the update. Yes, you can turn the old SH2 hub on without internet. Wait long enough and get the pinkish light. At this point you can de-resister the phone. Phone reset does indeed seem to do the same. After turning the SH2 hub off, I have been able to sync to a legacy DECT base. Functionality is really poor and I wonder if it really meets DECT standards. The phone won't let you get to DECT settings. You can't name the phone. It won't even show the time. But, it does work for outgoing and incoming calls and sound quality is fine.
All-in-all a very poor situation. Industry standards need setting up for domestic providers for digital phone solutions, to at least match DECT standards. So when I switched between ISPs, I don't find the standard hardware I have (like the phone) is rendered impaired or completely dysfunctional.
That's exactly as I'd expect. There isn't a DECT standard but rather a Generic Access Profile that requires handsets from different manufacturers to work on other base stations. But as you've found, that's at the most basic of levels. You would find the same constraints between any two manufacturers & probably even between ranges by the same manufacturer.