Dual Sim working but a question


    I have a OnePlus 7 Pro - the unlocked version. I have 2 SIMs installed, one Tmo, one Verizon. Work pays for Verizon and it gives me a separate work line and other coverage options.


    I have noticed the following and wondering if anyone can explain why Tmo works the way it does.


    My configuration is as follows:

    Tmo SIM number 1

    Verizon SIM number 2

    Data on Verizon SIM


    Dual 4G networks is on - this says it will enable 4G network for both SIM cards when available.


    So with the above configuration, I noticed that the Verizon line is doing VoLTE, and Tmo line is 3G.


    If i switch to make Tmo the data line, then both Tmo and Verizon show VoLTE.


    So, first question is why - Verizon can do VoLTE when data is on Tmo, but Tmo can't. I am assuming TMo has some settings that force it to 3G instead of trying to do LTE. 


    I am not unhappy, as Verizon data, that work pays for is being used. and I am able (yes able) to do MMS from both SIM 1 and SIM 2 with data on Verizon. My only oddity is sometimes when texting on Tmo SIM, alls to TMO Line, go straight to VM. I also noticed that I don't have any kind of VoWifi at all.

      All replies

      • gramps28

        Re: Dual Sim working but a question

        Everything I've ever read when looking at dual sim phone is that sim 1 is voice/ Lte data and sim 2 is voice/2g

        but the OnePlus 7 may be different.


        I watched a video on OnePlus's website and that's how they showed how to set it up the dual sim.

          • syaoran

            Re: Dual Sim working but a question

            My Xiaomi Mi MIX 3 can do dual VoLTE.  However, most devices, like the Mi MIX 3 and OnePlus 7 Pro, they are dual-standby, which means when one SIM is in-use for a call, the other for calling is not. 


            The implementation for T-Mobile's VoLTE uses your APN settings, which is why it needs to be set to the active data line.  Verizon's CDMA allows for a higher call quality in general but more than likely do not actually push it over LTE, like with what is known as HD Voice.

          • gobigmagenta

            Re: Dual Sim working but a question

            Yes phone is Dual SIM Dual Standby, but many phones do support Dual VoLTE - many countries now are VoLTE only so if you didn't do dual VoLTE, the second line would not even be able to do standby.


            As for Verizon - HD Voice IS VoLTE - HD Voice is not CDMA and even Verizon is retiring CDMA next year and new phones from Verizon are CDMA-less. Phones that are not Verizon branded need to be added with a CDMA-less switch on the Verizon side.


            I guess I just have to wait for Tmobile to evolve to support VoLTE without the data setting in the APN. Not that it really matters now, but without VoLTE on Tmo, it appears i can't select VoWifi either. Not a show stopper, just wondering why.

              • syaoran

                Re: Dual Sim working but a question

                What version of Oxygen OS are you running?  Wifi Calling works fine on 9.5.11.

                  • gobigmagenta

                    Re: Dual Sim working but a question



                    I am running the latest version of Oxygen OS - but i suspect you are not using dual sim with Verizon SIM as the data SIM. Try Tmobile SIM without Data and see if Wifi calling appears.

                      • syaoran

                        Re: Dual Sim working but a question

                        I do not have a Verizon SIM, which could be the difference.  The only other SIM's I currently have access to are other T-Mobile SIM's and a Canadian Telus SIM.  Telus/Bell being roaming partners with T-Mobile won't really play any different.


                        If WiFi Calling disappears when another SIM is inserted with the T-Mobile SIM and the T-Mobile SIM is set as the primary SIM for calling.  Then it sounds like a bug.  When the option disappears, try putting *#*#4636#*#* in the dialer. Check Phone 1 and Phone 2 to ensure WiFi Calling is enabled.  Again, I don't have a Verizon SIM but the WiFi Calling provisioning should not change for the T-Mobile SIM.

                  • drnewcomb2

                    Re: Dual Sim working but a question

                    While I don't have an answer, the subject of dual-SIM phones interests me. Generally, on dual-SIM phones one of the SIMs is treated as a 2nd-class SIM. On many phones this means that SIM #2 is GSM only. This would completely negate exactly the sort of configuration we're now discussing (one Verizon, one T-Mobile). Ideally, one would like both SIMs running LTE on any bands simultaneously.


                    I actually looked at the OnePlus 7 Pro user manual and found nothing germane to this discussion. They don't even show the dual-SIM carrier. The whole topic is treated as an OBTW.


                    You might get a better response from the forum on OnePlus' support site.

                    • gobigmagenta

                      Re: Dual Sim working but a question

                      The onePlus forums have no more info - i tried there first.


                      I don't mind the dual sim dual standby - and I think because Verizon has NO GSM 3G, they have no choice but to offer VoLTE even when that sim is NOT being used for data - it seems to be a counter-intuitive as Data is used for VoLTE - but apparently the radios are able to do VoLTE even when there is no full data channel. Again, many countries skipped 2G/3G or retired it in favor of LTE.


                      This is also similar how Tmo pushed out band 12 (I think it was a few years ago) and used it for voice. If your phone didn't do band 12 you could show signal in some areas, but not be able to make a voice call. This was a concern for emergency services - sounds like it is related somehow.


                      I was just trying to figure out why Tmo drops to 3G when it has no data, but Verizon is able to provide VoLTE when it has no data.


                      TMobile version of the OnePlus phone removes the DualSim capability, but the USA variant sold on OnePlus does have it.


                      Anyway, i get a great tmo discount, and verizon is paid for by others, so all is good. I still get MMS on both lines, so I am happy, just curious.