The phone models were specifically mentioned in my post
The SIP helper won’t help here: previously tested and ruled out
After further research this seems to be an ADSL issue rather than a modem or router issue
This week in fact we have reduced, as per recommendations perused elsewhere, the “Subscribe Period”
(also known as “Session Timer” or “Login Expire”) from 1800secs to 240secs
Some even recommend a more aggressive setting of 120secs
So far no more disconnections, but it has only been three days