PBX on AWS Private or Public Subnet

Hello,

I had a few questions regarding the setup for a PBX setup on AWS or any cloud provider for that matter. Currently we have a CUCM environment which needs out phones locally connected to an on prem call manager.

We were looking at a new PBX solution hosted on a cloud provider but are struggling with some understanding of the fundamentals of setup. Should our instance be hosted on a private subnet behind nat (does it need any local resources) or should I just flat out put it on a public network with no NAT (do phones work through NAT?)?

I'm probably making this more complicated than it should be but obviously a public facing pbx would be easiest I just want to get the temperature of some more informed souls.

Thanks!