Caller ID on FXO Trunks (also GSM Gatewways)

I currently have a single SIP trunk and am considering a few backup trunks which can also cut my costs, however I am concerned that calls going out over FXO lines (Virgin Media) and GSM lines (Vodafone) may transmit the CID of the provider and not my main number. I know that there are a number of places to set asterisk to send a specific CID, but this does not seem to work on my SIP trunk at the minutes so I am wondering if adding more trunks will only multiply my problem.

Where is the best place to set the CID?

Are there any issues with changing the CID for FXO & GSM trunks?

I would be interested know of your experience before I spend.