safecam,
I take it you are using analog trunks? We have done this by putting the first line in a different outgoing group (maybe 1), then create a shortcode to dial the voicemail code and specify the new new line group id. eg
SC *99
Dial 123456 (or whatever your vmail code is)
LG 1
Feat Dial
If you still want users to access the line for normal calls then you need to invoke LCR to use this trunk as a 1st or second choice.
Hope this helps.