Anyone know Asterisk?

I have a client running their business telephony using Asterisk on a Gentoo Linux server, and they need to change providers because their old provider is going out of business. I have tried to set up a new SIP trunk service on their server and also on a test Asterisk installation I set up on my own Debian system, but I can't get calls to work either inbound or outbound. Anyone out there know how to configure Asterisk? Thanks, Andrew