Faking Caller ID
Any voice over IP gateway that uses an ISDN PRI interface will allow you to configure any calling number (caller ID) you like, and then signal it to the PSTN via the PRI during call set-up. The ability to control caller ID is necessary to seamlessly integrate VoIP endpoints (e.g. IP Phones) into the PSTN.
A Cisco 3620 would do the job, is 2" high by 19" wide and can be bought on Ebay today for $849. There is good, free 3620 configuration advice on www.cisco.com. There are likely cheaper alternatives, I just know setting caller ID can be done on a 3620.
T1 PRI pricing is dependent on your distance from the central office and whether you have a competitive alternative to your local RBOC, but can cost as little as $300/month. An E1 PRI will work just as well overseas. (From Risks)