thank you joesf - that has solved my problem completely!
In case anyone else is interested, the following was posted by chaplain in september 2011 and i've just followed the instructions which has increased the ring length time from 15 to 30 seconds.
on most droids, and most carriers:
1. Dial *#61# and click send. You will see your vm access number and the current ring delay time.
2. Write down the phone number displayed (including +1).
3. Dial **61*+1xxxyyyzzzz*11*30# and hit send. +1xxxyyyzzzz is the number you wrote down previously, 30 is the delay in seconds. The delay can be set in 5 second increments, 30 is maximum.
4. Dial *#61# to verify that the new settings are active.
i had looked for relevant threads, but not back that far, so many thanks for finding it for me