Can anyone explain why I see Dual VoLTE when Sim configured one way and not another

[chromedome00]

Specific to dual sim with Tmobile and Verizon - Unlocked version 7 pro:

When Tmobile SIM is data sim, both lines show VoLTE in status bar, and 4G under each sim. When Verizon is the data sim, only Verizon sim shows VoLTE, and Tmo is UMTS. Could this be a provisioning thing on tmo side? They see my phone registered as 7 Pro, but Tmo billing site shows unknown device.

Or does TMO require data for VoLTE and Verizon is looser allowing VoLTE when not on data because they have no 3G to fall back on?

Just wonder why Tmo falls back to 3G and doesn't use VoLTE when it is not the data sim? This implies there is something better about falling back to 3G when possible - maybe battery life or coverage?

 

[FSOP]

I'd also like to know this if anyone can explain

 

[dullerm]

Acknowledging that this post is rather old but I am facing the same issue. I have the issue with my 2 T-Mobile lines that I'm using in an unlocked OP7Pro. No chance to get VoLTE or VoWiFi going for a line unless it is the active data line. Given that T-Mobile wants to go LTE (and 5G) only early 2021 they should get this sorted out...
@FSOP @chromedome have you found out anything more on this topic?

 

[tech_head]

Specific to dual sim with Tmobile and Verizon - Unlocked version 7 pro:

When Tmobile SIM is data sim, both lines show VoLTE in status bar, and 4G under each sim. When Verizon is the data sim, only Verizon sim shows VoLTE, and Tmo is UMTS. Could this be a provisioning thing on tmo side? They see my phone registered as 7 Pro, but Tmo billing site shows unknown device.

Or does TMO require data for VoLTE and Verizon is looser allowing VoLTE when not on data because they have no 3G to fall back on?

Just wonder why Tmo falls back to 3G and doesn't use VoLTE when it is not the data sim? This implies there is something better about falling back to 3G when possible - maybe battery life or coverage?

I think it has to do with Verizon only has LTE on this device, so no matter what the Verizon connection is LTE.