Hi Debbie,
You can lengthen the time to a max of 30 seconds by entering this code :
**61*901*11*30# SEND (30 is the number of seconds.You can substitute this in increments of 05 seconds
eg 05, 10, 15 etc.)SEND means call as in dialling the code
You can also switch voicemail off by dialing 1760.
Hope this helps.