My 9000 bold has had this problem ever since i bought it-

it rings 4 times and hangs up-

how do i make it ring longer-

Why is that?

Thanks

Not sure why it rings and hangs up instead of going to voicemail (if that's what you mean), but that might be a question for your carrier.

As far as increasing the number of rings:

Increase amount of rings before voicemail - BlackBerryFAQ

That should help...