Hi!
There can be more than one reasons for that problem...
- You might have forced the CID on your route/trunks.
- Your provider might block you from using a CID you don't own and are forcing you to use yours.
and there are probably others I am not thinking of at this time..
Look at your routes and trunks and ask your provider if they are the ones forcing you to use a specific DID...
Good luck and have a nice day!
Nick