It's good to know that someone else uses a 10-year-old phone!
The instructions for cancelling all diverts are on pages 88 and 89 of the manual:
http://nds1.nokia.com...310_usersguide_en.pdf
If those don't work, there might be a fault with the phone. Try turning it off, waiting a short while, and then turning it on again.
If that doesn't work, try it again but take the battery out, and then replace it, when the phone is turned off.
If that still doesn't work, try restoring the factory settings (see pp 80 & 87). If you're asked for a security code (and you've not previously entered your own choice) it's 12345.
Chris