When making an outbound or outgoing call the receiving party will see a caller number from yourself.
If you have multiple numbers, you can pick which one will be used to present to the called party.
Please note that you can only present the numbers that you actually have on NUACOM account. If you wish to use as a Caller ID a number that is not currently in NUACOM, you will need to port it over to NUACOM first.
The caller ID can be set per extension, to do that please log in to the NUACOM portal then go to Settings > Extensions and press on the extension you wish to edit.
From the "Caller ID" drop-down pick the desired number and press "OK".