As a test, temporarily set Outbound CID for the extension to the failing caller’s number and see whether it shows correctly when calling his mobile from the extension.
If that also fails, check whether the CDR for the test call shows the correct Outbound CallerID.
If Outbound CallerID is wrong, check Outbound Route and Trunk for (unwanted) overrides.
If Outbound CallerID is correct but doesn’t show on the mobile, there are several possibilities:
- Your trunking provider may not allow you send a number that is not yours, or may require you to sign a supplementary agreement to permit this.
- You are sending headers (P-Asserted-Identity, Diversion, etc.) that differ from what the provider requires.
- You are sending the calling number in a format other than what the provider requires (country code, initial +, etc.)
A good bet is to try the same headers and same format as they send you on incoming calls.