My 4G package arrived yesterday. The quirky/goofy 4G module I had sent back had been replaced by a SIM7600A for the USA. It is no longer quirky, and after considerable experimentation, I think the secret is revealed. Read onā¦
The 2G module supports only GSM, and T-Mobile/Tingās GSM service is terrible in my area. This particular 4G module (SIM7600A) does not even support GSM; however it does support LTE and (as I have now learned) UMTS (AKA WCDMA, a successor to GSM). T-Mobile/Ting supports GSM (poorly), LTE, and UMTS. Their UMTS signal is Band 2 (1900MHz) just like their GSM signal, but is somewhat stronger and does penetrate all but one room of my house (you can key in your address on the Ting coverage map and see exactly what modes are offered where you are.) Furthermore, just about anywhere other than my house the UTMS signal is very strong. So there definitly is an improvement, but it has nothing whatever to do with LTE.
What I did find is that if I forced the SIM7600A to LTE only mode, it would not acquire a signal from T-Mobile/Ting even though they utilize LTE bands 2 and 12 extensively and Iām sure the signals are there. I take this to mean that VoLTE is not supported by the SIM7600A (its documentation has virtually no info on VoLTE.) I could not get a Verizon SIM card to work in any way, shape, or form, so Iām assuming they donāt support UMTS.
How did I discover this? (you may ask)ā¦ it turns out if you follow the settings menu to the āSIM Module debugā option (you can skip the List Operators) and plug your phone into a computer running Arduino IDE (and set the port right), you can open the Serial Monitor and type in commands to the SIM7600A (with line ending set to Carriage return).
The command AT+CNSMOD? requests the current cellular mode. With the Ting card connected to the network, I get a response of +CNSMOD: 0,4, which means itās connected UMTS/WCDMA. If Iām not connected to the network, the response is +CNSMOD: 0,0 which means disconnected. If the second number were 8, it would mean connected via LTE, but that never happens.
The command AT+CNMP=14 sets the SIM7600A to connect with UMTS/WCDMA only; That works fine. The command AT+CNMP=38 sets the SIM7600A to connect with LTE only; That never results in a connection until I send AT+CNMP=14 again or AT+CNMP=2 (automatic).
So the bottom line: my point is that VoLTE just doesnāt work at all and it appears that Verizon (offering CDMA and LTE only) doesnāt work (the SIM7600A does not support CDMA). But if you stick with T-Mobile/Ting you should get reasonable reception over UMTS which is selected automatically by the SIM7600A, at least here where I live in southeastern Pennsylvania. I canāt explain the rumors of folks in the USA using Verizon successfully, but Iād be interested in what reply they would get if they sent the AT+CNSMOD? command.
In fact, if everyone who reads this should try sending that command and reporting back here what the reply is, we could expand the knowledge base regarding this issue greatly.
For reference, these two commands, and hundreds of others can be found in this document:
https://simcom.ee/documents/SIM7600C/SIM7500_SIM7600%20Series_AT%20Command%20Manual_V1.01.pdf
One last thought regarding VoLTE. Although itās virtually undocumented in the SIM7600A documentation, there are smatterings of information on the internet that seem to imply that VoLTE does exist on this chip but needs to be enabled by some hocus-pocus burning of something undocumented to the chip. All very strange.