My 8310 (T-Mobile) is forwarding calls automatically already after 5 rings. Is there a way to lenghten that time, say after 10 rings?

You can extend it to 30 seconds.
Increase amount of rings before voicemail - BlackBerryFAQ

great tip - it's device independent and needs to be looked up with the carrier.
For T-Mobile Germany the string you need is
**61*3311*11*30#
Weras the last two digits (30) indicate the delay in seconds until the call is forwarded.
T-Mobil obviously recently changed the default setting - you barly had time to grab your phone before calls were forwarded.
Thanks