This solves 2FA codes, which was indeed part of our problem, but it doesn't solve incoming/outgoing calls that ideally needed to be on the same number as well for when we dealt with humans.

This is probably possible to do, but probably hard to get right, and still requires having a device reliably available to receive calls, and has limited scale (what happens if there are multiple calls at the same time?). This is why it would have been great to be able to buy this as a service.