Or to be really pedantic, 'is currently assigned a particular' phone number.

Since you can change phone number without changing SIM (I don't know if it's global, but in the UK you just text a certain number for a transfer 'PAC' code) and clone them.

It's harder than that in the US, but it's possible. Mostly it depends on the carrier's policy.

Or SMS. Calls may also work, depending on the modem.

For calls and sms you'd need a client configured for that modem. I think this is what callcenters use?

SMS are easy, you can do that with AT-Commands. Voice calls, I don't know, haven't tested that. ModemManager has support for it though, and for SMS.

https://github.com/freedesktop/ModemManager/blob/master/NEWS