SIP trunk Problem

I have credentials from local voip server and that is only for incoming and I can connect through xlite and can received calls but when I want to peer my asterisk (1.2) with particular server it return (Sip_req_timeout) error another thing my service provider does not use default sip port instead it uses 3128 and when I enter at cli “sip show registry” it shows “ip:3128 request sent” is it port problem or any other issue and xlite is running fine no problem.
your help is required
thank you

My configuration is as follow

SIP.conf

Register => username:password@ip:3128
type=user
Username=test
Password=test
Allow=all
Host =ip address:3128
Context=abc

Exten.conf

I make appropriate dial plan
My distribution is CentOS 4.3 and service provider I know nothing other than username and password