EC500 will send the original Caller's ID to the mobile phone. However, many SIP trunks do not allow outbound calls with an ORIGINATING Caller ID that does not belong on that trunk, so the call is denied by the carrier. This is pretty normal for SIP trunks.
One thing you can try is on the Adaptation for the SBC is enter the circuit billing number into the Adaptation Data field for Outbound calls from SM to the SBC. You may need to contact your SIP provider, too, to make sure it all works correctly. The provider can tell you exactly which number to enter if you don't know the billing number.