FreePBX is showing the number exactly as the Mediatrix is presenting it. If the Mediatrix is managed by your provider and not you, you could use a specific context to modify the CallerID that you are getting from the Mediatrix in order to add the missing 0, but it could get tricky if some numbers should have it but some others shouldn’t.
I’m not sure exactly which model you have, but I bet the documentation for it will explain how to do that.
Then again, are all numbers supposed to have a 0 at the begining?