Two or three big suppliers deal with this kind of equipment in Islamabad. For Panasonic the dealers are Mansha Brothers. There web site link is:
And for Nortel the dealers are Appolo telecom ; I dont have their web site but the local numbers are 2651361, 2651362
If you need cheaper chinese brands look up or ask for ZTE. The basic chinese models with vendors usually donot have a PRI interface.
I will recommend a Nortel Meridian or a Panasonic Hybrid exchange. BTW The yellow pages of Paksitan are at : http://www.yellowpages.biz.pk/
Hope this helps.