phone number unknown

Had the same issue and was unable to set up my voicemail because in the About Me of my Phone's settings, the number was listed as Unknown. I finally found the solution and thought I'd share it with you. To be able to set up voicemail, this is what you have to do: Tap the phone app (main utility app for making calls, is usually on the bottom of your screen, has a phone on it..) then on the search bar at the top, click the three dots on the right prompting a dropdown menu. From there, click 'Call Settings'... you'll be taken to another list and click 'Voicemail'.. taken to another list and click 'Setup' and here you can manually entered your phone number! Upon doing so, going back to the phone app and dial 1, and you are now able to set up Voicemail. Hope this helps! :D

Update: Tried this solution for my new phone and didn't work this time. :/ Went into the Voicemail, Advanced Settings, Setup, and input my new number... tried it with my country code (1), and then without the country code (only area code) and restarting it after these changes had been made each time and then tried to dial 1 but it keeps saying 'Service Code not Supported'. No voicemail? Uh oh. That's not good. Any advice AT&T Prepaid?  This is the Alcatel Insight, btw..

Hello Forum, I also have a new service where my phone does not know it's own phone number. I received a startup packet from AT&T (auto pay) with simm and literature including my new phone number. I installed the simm in an unlocked Galaxy J3 Orbit (previously Tracfone). I am able to use wireless to browse the internet and tether my laptop to the internet but I have no phone service in or out. It hangs up as soon as the number is dialed. I removed, cleaned and re-installed the simm a couple of times with numerous full shutdowns and still no phone. I also seem to be getting someone elses text message sporadically. Any ideas? Thanks, Kerry

I have the same issue on the new MIcrosoft Surface DUO.  I tried chatting with different ATT reps and it includes 2 supervisors.  One of them suggested I file a warranty claim? and the other one said, just return it?  I was like [EDITED]1500 phone and it doesn't register the number?

[EDITED per Community Guidelines]

I took my phone to the AT&T store in Pittsfield where after an hour and a half, with the help of a tech, and the help of a few other techs, and a call to tech service were all unable to get to the bottom of it. I purchased a new phone to put everyone out of their misery which, of course, fixed it. Kerry

I'm currently having this issue and it's not the phone. I've had AT&T prepaid for several years and I just recently got a new SIM card because I needed to change my number and it's cheaper that way rather than being charged $25. Same phone, new SIM card and now this is an issue. The problem is them and their SIM card! 

I walked into an At&t store, they changed the SIM twice and the 2nd one worked. I told them if it didn't work I was returning the device. Sure enough, it worked.  Make sure they activate the sim cards on each attempt.

I just got and activated a prepaid AT&T Samsung Galaxy A12 and cannot make a login for the MyAT&T app it doesn't recognize the zip or phone number buuuuut the zip is the same as the store and my number works fine so idk what to do.