Hi,
you need to setup 2 ser servers if u want to use voicemail.
One Server as "regular" sip proxy and one ser-server as dedicated voicemail server (on a different port if on the same machine).
Then you route the calls from the "regular" ser server to the dedicated voicemail server if the call isnt answered after a certain amount of time.
You cant define the timeout in means of "rings" because the ser server doesnt know how often an UAC will ring.
If you search the mail archive you will find several example configs for this setup.
best regards,
Arnd