If you are looking around “free calls to Pakistan” that is almost impossible, there is nothing really for 
free. But yes there are plenty of VoIP termination
competitors which can help. However here is some info from “evongetz” that may help you to try some cheapest way to calling Pakistan through SIP.
The Session Initiation Protocol (SIP) is a signalling protocol used for establishing sessions in
an IP network. A session could be a simple two-way telephone call or it could be a collaborative multi-media conference session.
The following Carriers give SIP accounts,
Send someone to their office and get an account, and configure it in your switch as a route, and start sending traffic. Read the rest of this entry »
Tags: free calls to pakistan, PTCL, telecom, telephone call, voip market, wateen