Yes, your SIP trunk provider can give you the list of DID's that are assigned to your main number. That info (or at least the ones that are assigned) should be accessible in the phone system you have as well in the administration portal. A DID (Direct Inward Dial) is really a way to dial someone directly rather than having to dial the main number and enter their extension. It is not tied to the number of lines (trunks) you have. Typically you will purchase a block of numbers from the carrier and assign them to users in the phone system (e.g.555-1234 maps to extension 102). So technically you could have 23 lines (trunks, channels, call paths) and 100 DID's.