A big thank you to everyone who wrote about their travails with AT&T support. Clearly, they don't know what they are doing, and ended up being a big waste of time (that I got to skip, God bless you all!!!! :-D
NOTE: I DON'T HAVE ENOUGH POSTS TO POST LINKS. GAH! Sorry!
It is NOT limited to the Samsung phones. I have an LG-G3, with AT&T, with the same problem. AND, I'm updated to Lollipop (5.0.1).
Sooooo, it's not Samsung's fault, and Lollipop hasn't fixed it in all cases :-/
In my case, I found out there was a problem when I tried to get Google Voice to work (for vm only, for my existing, AT&T number). Ironically, I've not had this problem in any of the previous 4 LG-G3 phones I've had while using Google Voice. (Yup. I'm on my 6th [dang] LG-G3, plus multiple factory resets and SIM card replacements.....)
I've meandered around various forums trying to figure out why I couldn't get Google Voice vm to work for me. One issue I have is that it tries to sign me in to the wrong gmail account. However, I've worked around that (don't remember how) and the phone indicates that the proper gmail account is associated with my Google Voice app.
My phone is set to GV for "Service" so it's not that. (Settings then Phone then Service then GV)
One of the workarounds mentioned (here and elsewhere) was to turn on Call Forwarding. Unfortunately, this is where I get the SIM card network error message. I also get error messages when trying to activate the GV number. I.e., when trying to send *004*15555555# through my keypad (where 555555 is in place of my GV phone number). Following the number given to deactivate likewise didn't work, and resulted in the same error.
I've not been able to figure out how to add time to before the built-in service answers the phone (to fix the SIM card error notice), as it didn't work due to the SIM card error. (I coulda had a cool lin.k here. Sorry!!!)
There was another solution (to the GV problem), which was to set the phone to have a delay in answering (in order to give GV enough time to answer, instead of the built-in vm). Alas, I still got the MMI error, so it didn't work for me.
I did, however, achieve success (though with a feature I'd NOT wanted), using the workaround of screening calls through GV. (Perhaps it serves the same function of the delay, i.e., answering the phone before the built-in one does??) Unfortunately, this means callers are told to give their name before I answer --not exactly client-friendly!
PS I also tried using codes mentioned in this thread, I think?, to give a delay in the phone automatically answering. However, I got the same [dang]MMI error code.
[It had to do w/61. Evidently I can't post that either.)
I hope this helps (at least a little!).
All the best