You can specify your CallerId in the admin center. Login, then choose "options" under the TELEPHONY section, then choose "caller id". After that specify your CallerId which will be used for your outgoing calls and SMS for this account.
Sometimes, CallerID which is set in your Softphone or IP phone is not passed to the destination phone number due to the way the telco industry works. At Diamondcard we pass the caller ID on but depending on the local telco that terminates the call it may or may not display on the destination phone.
Most IAX2 clients allow to configure CallerID number. Thats the field that needs to be set if you want to override your CallerID.
SIP devices usually don't have special field for CallerID. With Diamondcard you can specify phone number which you want to be presented as a CallerId number in a "Name" field.
This is a field where you usually specify your name. If you add a phone number after the name it'll be used as a CallerID. This field called differently on different SIP devices. "Display Name" in Lynksys, "Your name" in Twinkle softphone, "First Name" + "Surname" in Ekiga.
Here is an example: "John Smith 13031234567". 13031234567 will be used as a CalledId number.