With GSM phones, you can usually program these things using the GSM Control Codes. Search the web for code sequences specific to your carrier. For example, I use Cingular and set it to wait 30 secs before forwarding to VM.
Cingular VM Rings:
1. Note the tel # that your Voicemail uses, including 1xxxyyyzzzz (all digits including 1).
2. to set your delay, dial
the number 30 above can be any value <= 30 and is the delay in seconds before the call goes to voicemail.
Here's a site I just found that has many codes, there are dozens of sites like this that show diff codes. Good luck. GSM codes - change settings on GSM mobile phones