Assuming you have PRI trunks or SIP trunks (won't work with Loop Start, of course), you can change the CALLING PARTY NUMBER in individual phone programming. This is the number that will be sent as outbound caller ID. BUT some carriers don't allow you to send different numbers, and other carriers turn this off by default. Try changing the number, see what happens. If it doesn't work, check with your carrier. There's also a FORCE TRUNK GROUP CALLING PARTY NAME AND NUMBER in the trunk group, should be NO, and is NO by default.