[McMurdo]

There's a BT engineering note that says that the exchange line(s) must be present when powering up otherwise extension 20 and 21 will 'lock up' and not work properly. Might be the case?

We had 2 BT2+8's here (mk1's) and I got sick of their quirks and random glitches, so installed a Panasonic and never looked back.