It would be nice to have an Option to manipulate the CALLERID(num). For example: Calls form a special sip trunk should be reported with the prefix 99 from the other trunk 98. This may be important for CTI-Softwares. Where you cant use the CID name prefix to idetify.
An other method like in capi.conf would be also good. Something like an prefix=99 in SIP Trunk Configuration (Incoming Settings)
You can do that by setting the context= statement to point to a custom context in extensions_custom.conf, then in the custom context add the prefix you want, then send the call on to from-trunk. See
http://www.freepbx.org/support/documentation/howtos/how-to-change-incoming-callerid
If you actually want an option to do Caller-ID manipulation of incoming call, you might want to use the bug tracking system and make that a feature request. I know the FreePBX devs have talked about doing a rewrite of trunks and routes (they are the parts of FreePBX that seem to give the most people fits) so maybe they could include something like what you want, if they are aware that people want it (note I have NOT looked to see if there is already a feature request ticket open that asks for this feature, so you may want to check on that first).