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.

The typical cost of this will be 1 cent a min ( 60 paisa per min to land line) to Pakistan.

The following companies give SIP accounts, and you can buy them, and configure them in your switch.

Wateen www.wateen.com
BrainTel www.braintel.net.pk
Telecard www.telecard.com.pk
WorldCall www.go4b.net.pk
Hazara Telecom www.hccnp.net.pk
Burraq  http://www.burraqtel.com.pk

Soon Link.net and PTCL.

Wateen and Burraq:
Get a Wimax box, and get a local phone line with it.

Then email to Wateen and ask them the sip password for it.
They will send you the SIP configuration, If they dont, search on google, on how to get out the sip password out of wateen box, u will find a LOT of links.

Configure this sip account in your switch as your route to Pakistan and enjoy the 60 Paisa a min or 1 cent a min service from them.

Enjoy !!