What equipment would you like to use at the UAE end (OBi, IP Phone, smartphone app, softphone, etc.)? I'll assume a softphone, since that is simplest. What kind of VPN (OpenVPN, PPTP, L2TP, IPSEC, etc.)? Hardware-based, e.g. built into your router, or software, e.g. VPN connection on Windows, Mac, Linux?
If the VPN server provides you with an unrestricted public IP or a "friendly" NATed address, you should be able to run either OBiAPP+softphone or a SIP softphone in UAE, with a corresponding OBiTalk or SIP connection in Pakistan. Using OBiTalk is simpler and will allow one-stage dialing, but may be less reliable and lower quality. In any case, I suggest that you test the proposed path in software, before buying or installing any equipment.
Without any setup at the remote end, you should be able to call Pakistan quite inexpensively. With your existing VPN connection, can you make calls via Gmail? Skype? Yahoo Messenger? You could also try a SIP provider such as Callcentric (IMO top quality, $0.0385/min. to Pakistan mobile) or CallWithUs ($0.018). These services can be tested without making a payment, e.g. free calls to US toll-free numbers on Gmail, Skype, Yahoo, or calls to test numbers on the others.
If you can't get VoIP to work, but have a landline or mobile in UAE to receive calls, a click-to-call or callback service could still save you some money. For example, on Callcentric you'd pay $0.1485 to UAE mobile + $0.0385 to Pakistan mobile, totaling $0.187 for the call.
