I've been using the same sim card for a few years with Tmobile (same ZTE android 5.1 phone). And about 2 weeks ago (I work in Canada crossing the US border often) I started getting a "sim not allowed MM #3" error message on my unlocked phone. This results in no phone/ network service whatsoever.
I pulled the sim card out, wiped it and restarted. It worked ok for a day or so and the problem is back. I cannot go for more then 4 or 5 hrs with the error message coming back (while in Canada). Or I simply get a message saying "no service". If I try to re-connect to the mobile network my sim just refuses to do so.
As a fix I have to do a reboot of the phone to get reconnected to the canadian network that tmobile partners with here (telus). Also I check the APN number and reset that to default so no issues there. The IMEI/ESN is not on any blacklists...besides I've had the phone for a few years as I said.
I've got lots of credit and Tmobile plan does not expire until Oct 2018.
Can someone offer some help?
Wow, I've never heard of that error popping up only while roaming. It sounds like it may be a provisioning issue. Have you tried speaking with Tech Support yet? If you don't have the ability to make/receive calls in Canada at the moment, you can work with our T-Force team via social media using the links in my signature or by going here.