The email2:UmCallerID element<21> is an optional element that specifies the callback telephone number of the person who called or left an electronic voice message. It is defined as an element in the Email2 namespace.
The string ([MS-ASDTYPE] section 2.6) value of this element is either formatted as an E.164 telephone number, as specified in [E164], or as a session initiated protocol link to initiate a Voice over IP (VoIP) call. For more details about session initiated protocol links, see [RFC3261].
This element is sent from the server to the client, and MUST NOT be sent from the client to the server. The server returns a Status element ([MS-ASCMD] section 126.96.36.199.16) value of 6 in the Sync command response ([MS-ASCMD] section 188.8.131.52) if the client attempts to send the email2:UmCallerId element to the server. The email2:UmCallerID element is not included in the command response, or is empty in the command response, if the call originated as a private, blocked, or otherwise anonymous call meaning the caller’s telephone number was masked and unavailable to the recipient of the call.
This element MUST only be included for messages with one of the following MessageClass values:
Only one email2:UmCallerID element is allowed per message. In order to enable future VoIP scenarios, the server SHOULD send this element to the client regardless of the client's current VoIP capabilities.