I have BT Premium Trio (+ 1 additional phone) and just recently, the main base has been beepinn twice intermittently. I have not yet identified a specific gap between beeps but it is certainly over 10 minutes.
There are no outstanding messages on the base answerphone so it cannot be the standard message alert. Anyway, that is turned off via the handset.
If s/w versions help, I have got this fro. The handset and the base is