Default [Q] Error Invalid Number text whenever I reboot

SOLVED: This was being caused by the T-Mobile Visual Voicemail app. Disabling the app fixed the problem.

I recently switched from T-Mobile to AT&T, bringing over my unlocked T-Mobile Galaxy S4 (not rooted, running stock android, got the SIM unlock code from T-Mobile). After fiddling with the APN settings everything seems to be working fine, except every time I reboot I get the following text message:

From: +1121611611
122 Error Invalid Number. Please re-send using a valid 10 digit mobile number or valid short code. Activate:dt=15

Any ideas? I first saw this error text the first time I rebooted my phone after switching the SIM card to AT&T, and I get the same text whenever I reboot my phone now.