How to replace a missing Http header bu using F5 ?

The HTTP header enrichtment for MMS traffic is typically done on the WAP2 gateway, which is an network enforced HTTP proxy with a RADIUS accounting proxy, allowing to get all RADIUS packets from the GGSN as the PDP/IP conect for the client is established. Then the gateway enriches the HTTP headers with any RADIUS provided values, i.e. Calling-Station-Id woudl be your MSISDN for the MMSC. See Kannel WAP gateway for an open source WAP gateway that can do this.