Caller ID (ANI) can be overwritten in a few different places within the unit, but I would likely need to know more about the application by seeing the complete configuration and the debug output for a test call. Below are the typical debug commands we use.
debug sip stack message
debug voice verbose
debug isdn L2-formatted
Feel free to reply on this post with a "cleaned" version of your unit's configuration and the debug output and I would be glad to look it over. As one example of how this number might be changed, you can do a standard match and substitution on an egress trunk.
voice trunk T01 type sip
match ani 1000 substitute 5552000
match ani 1001 substitute 5553000
Hope this helps!