[Resolved---kind of] Cannot use self service short codes

[drhere]

I am getting a "connection error or invalid MMI code" error when I use any self service short code I am on T-Mobile. I rebooted multiple times, had my phone reconnect to the network booted in safe mode.
It was working normally for awhile, then last week it stopped. I called Tmo support and it has been escalated to a higher tier, but they can't figure it out.
I saw one site stating this is common for dual SIM phones, I have a Nokia 6 it is a dual SIM but US version of this phone, and I am only using one SIM.
Any thoughts on what can be causing this, I get SMS and calls on Tmo normally otherwise. Thanks.

 

[drhere]

Well, someone helped me resolve this (no not Tmo, but it wasn't a Tmo issue). Seems the Nokia 6 no longer switches to HSPA+ mode in order to process the short code (USSD) which it needs to do, the workaround is to temporarily toggle the Nokia 6 to 3G, once this is done, the short codes work. I can then place it back onto 4G (LTE).
It used to work, so I am wondering if a recent Nokia update changed this. Not a big deal, I just have to toggle to 3G use the short code, then go back to 4G, a few seconds.
This person stated he had an issue with his LG, and found a setting to allow USSD over IMS in the hidden menu to resolve this so he doesn't have to toggle back and forth. I did not see this option in the hidden menu, so the 6 may not support it.