Сам не игрался, но по-моему это то, что тебе нужно. Настраивается не через Obi, а через VOIP.MS, в подменю DID Numbers.

The DISA (Direct Inward System Access) let you dial numbers via your VOIP Account from Inbound Calls. It gives you directly the access to dial a phone number of your choice, similar to what a Calling Card service does. For example, you can create a DISA Entry, and configure one of your DID Number routing to point to a DISA. When you call, you will be prompted a PIN number and the telephone number you want to dial to. You can also use DISA from IVR's (Digital Receptionist), Time Conditions and CallerID Filtering.