Not a customer but am planning on moving to JMPChat. Like Google Fi, it's a VoIP (voice over internet protocol) service so coverage should be the same as Fi.
A data plan is available which will connect with cell towers but the company claims your location will always be obscured. I can't say how that is done. I use another voip service called voip.ms and just leave my phone in airplane mode unless I need to use my data plan. Encase you are wondering about voip.ms, there is no privacy with that service.
Can be purchased without any PII (personally identifiable information) and they accept crypto.