Shahzad,

I think PTCL/PTA is the governing body in Pakistan. You should check the respective websites for standards/specifications to be met.

For your application, you can use a PHILIPS TEA1062 chip as interface to line. YOu can use the MIC input of this chip to transfer your voice message to telephone line.

I will look for some info for you, but you should try a search for "Telephone line audio interface".

You will find some useful articles.

IA