Log into the admin GUI, and then go to Voice > Reports > Extension List
Do you see the extension listed there?
Also go to Voice > Stations > IP Phone Configs
Do you see the station associated to a phone?
If you still don’t see it then go into CLI enable mode, and do following
Voice user 7137
Then give us the error message if one or let me know it that worked.
After looking a bit more (and talking to Adtran Tech Support), it turned out we had a ring group set up with that extension, so that's why it was failing to save.
Good to hear! Glad you got it figured out.
Thanks for using the Forums and come back if you need any further assistance.