This has been discussed so many times here on support forums, you may do Google and will find plenty of info in detail.
Short answer of this question-- Yes, you can configure trunks and can use them for incoming and outgoing calls but make sure your organization doesn't violate TRAI rules (most imp rule- don't mix up pstn and sip calls), rules in simple words can be find on below link, but for proof you need to go through TRAI' website.
https://supportforums.cisco.com/blog/12539601/trai-india-regulations-voipip-telephony
Suresh
