I had already upgraded to the latest firmware are part of my own troubleshooting via the server, and initially it was stable, though the connection problem persisted. The mobile connection then dropped completely after about 24 hours and would not reconnect, via either sim port. I had to reboot the modem to get it to connect (no problem with the mobile service itself which was tested with a mobile phone at the same time). At your suggestion have now reloaded the firmware from a file downloaded from the Teltonika site, however the problem persists.
Rebooting the Polycom phone seems to solve it for a short while (anything from a couple of minutes to an hour or so) and then whilst I can dial out, anybody ringing in gets 'person unavailable'.
I should add that the Polycom is completely stable when connected wired directly to our domestic broadband router, which is why I'm suspecting the RUT955 as the source of the problem.