I am going to backup a little from the above answers and ask you, what kind of trunks? The answer will vary...
Copper, analog trunks and traditional T1 - Call telco, they provide CID on out bound calls
ISDN PRI & SIP - the default CID number is set in outbound characteristics in the trunk programming on a link basis, individual DIDs are set in the same area but a different form
With PRI, if you don't provide an outbound caller ID, the telco usually provides a default one, with SIP providers it often works that way, but some do not provide any CID if you don't send it specifically with every call.