Azure comms, multiple calling numbers....

Question

I'm developing a web application with integrated telephony. I'm using the voice calling SDK for calling over public switched telephone networks (PSTN). In the front-end of the app (React JS), I'm using the @azure/communication-calling library, and the line of code below initiates an outgoing call:

const call = await callAgent.startCall([{ phoneNumber: <numberToCall> }], { alternateCallerId: { phoneNumber: <callingNumber> } });

As the app is to have users in Ireland and Chicago, I have two numbers in the comms service: One for Ireland and one for Chicago. App users won't be calling each other, just clients ( members of the public ) in their own locality over PSTN. I originally assumed that the API would automatically figure out which of the calling numbers to use based on the country code of the recipient number, but someone in a reply to a previous post said it doesn't. In that case, how does one specify? Would specifying it in the alternateCallerId parameter in the above line of code be enough? Please advise.