What’s required to run an SMS subscription service in Pakistan?

You would need a short-code and need to be licensed by PTA (if you will be conducting business over SMS). You best bet is to contact companies that specialize in this, and the one name that pops up is VectraCom (see http://www.vxt.com/). They handle all that you want and provide this solution to you very easily, they pretty much work with all the telcos and provide this service to banks mobile carriers and other businesses.

The era of unlimited text messages would also soon be ending in Pakistan due to SMS spam.