VoLTE issues: Hackday article
Re: VoLTE issues: Hackday article
Does the Un-Smartphone have VoLTE? 
Re: VoLTE issues: Hackday article
I answered my own question: yes. 
https://skysedge.com/unsmartphones/RUSP/index.html
A: This is a fully-modern LTE device supporting Voice over LTE (VoLTE). 5G mainly adds mm-wave high-bandwidth service capabilities for extremely fast data rates, but this is not a useful feature for a voice-only cell phone like the Rotary Un-Smartphone. Because LTE is the backbone upon which 5G is built, the infrastructure for voice, text, and lower-bandwidth data transfer that LTE provides will exist for years to come. There's no disadvantage to using an LTE phone on the 5G networks, despite the wireless companies' seeming desperation in pushing 5G.
https://skysedge.com/unsmartphones/RUSP/index.html
A: This is a fully-modern LTE device supporting Voice over LTE (VoLTE). 5G mainly adds mm-wave high-bandwidth service capabilities for extremely fast data rates, but this is not a useful feature for a voice-only cell phone like the Rotary Un-Smartphone. Because LTE is the backbone upon which 5G is built, the infrastructure for voice, text, and lower-bandwidth data transfer that LTE provides will exist for years to come. There's no disadvantage to using an LTE phone on the 5G networks, despite the wireless companies' seeming desperation in pushing 5G.