External VoIP Providers

I have two VoIP providers one for incoming calls (they host our telephone numbers) and one for outbound. I have no trouble dialing out from my extensions, but cannot get the incoming to work. We have about 20 DID numbers with this company and need to get these numbers working into our Asterisk server, does anyone know of a link on how to set this up? Does anyone know how to set this up? I’ve been using Asterisk for quite some time, but have never used it without having ZAP T1 or PSTN interfaces. Thanks in advance.