SIP gateway?

Host: RHEL5
Version: 1.4.26

I am attempting to do the following and wonder if I am on the right lines. I have RT’d much of the FM.

I have a LAMP server sitting in a datacentre, providing an application.

I have a account and number.

I want an external customer to call that number. I want Asterisk to fire a message to the LAMP application with the inbound number.

Then I want that call routed to a PSTN number where the user is using the application.

I believe I need to do the following…

1: Configure sip.conf to make asterisk a SIP client.
2: Create a dial plan so that inbound calls from this number are routed to the PSTN number. Also execute some code at this point.

Am I on the right lines?

What I don’t understand I why I don’t need to configure Sipgate to talk to my server?