When implementing Enterprise Voice in OCS 2007 the assumption is that each user homed on OCS will have his/her own dedicated phone number directly reachable from the PSTN, a so called Direct Inward Dialing (DID) or Direct Dial-In (DDI). It is the DID defined for the user in E.164/RFC 3966 format in the msRTCSIP-Line attribute, i.e. tel:+4544890101. Some customers might not have a DID for each user. In stead they have 1 DID for the company, i.e. the main number, and all recipients have a unique extension assigned. An Interactive Voice Response (IVR) system or a switch board operator answers the main number and redirect the calls to the recipient.
So how could you implement Enterprise Voice in such a scenario? In a non-DID scenario you will have to define the msRTCSIP-Line attribute for each user in the format tel:+<main number in E.164>;ext=<extension>, i.e. tel:+4544890101;ext=200.
When implementing this type of solution you need to make sure that your SIP/PSTN gateway can understand this format. Why? It needs to be able to understand the components of the string and work with them. On outgoing calls from OCS to the PBX they need to be able to extract the extension from the Calling Line Id and present only the extension to the PBX (i.e. strip "tel:+<main number in E.164>;ext="). On incoming calls to OCS from the PBX they need to generate the format for Calling Line Id and Called Line Id.
We are looking at alternative solutions for this scenario for future releases of OCS 2007.