To have VoLTE or VoNR (the 5G version) the SIP based software handling your calls needs to interact directly with the software stack managed by the operator via a bunch of other telco protocols.
I don't think you will be able to support "regular" calls/SMS without if it is not supported by amazon.
Most of articles comparing 4G/5G and Wifi are focusing just on bandwidth and latency.
That is somehow like comparing cars in just how fast they can go.
3G/4G/5G are standardized and built for operators that are used to manage huge amount of users.
A lot of effort is put in avoiding congestion, providing guaranteed bitrate/latency for dedicated services (like your phone calls), smooth handovers between different antennas, a lot of of security features, and obviously, an excellent user tracking in order to charge the customers.
