Yeah, why complicate things? Already built into every iOS device, and functions as long as there is internet access. Just make sure it's enabled, and that the Apple ID address is checked/included as one of the "can be reached at" personal IDs. OP indicates no need for cross platform.
And if there is, most, if not all of the major messaging apps also provide for voice communication.
The cow is already in possession. No need to buy milk separately.
There is also a way to FaceTime Android users as well. You need iOS 15 or higher to do it.