According to this older thread a few years back people had this issue and they had to contact moderators to fix it.
To contact MODs see pic below.
Hello anyone at the @Moderator_Team
Can someone help me, please!!! In the last little while when someone calls me or I call them 3 beeps happen and disconnected after a 1 min. It is frustrating and annoying. Can some please help! @Moderator_Team
Two things to try:
1. Try the lost/stolen feature.
Log in to self-serve. Plan & Add-Ons from top menu. Lost/Stolen Phone from next menu. Suspend Service button. Log out. Wait a few minutes. Log back in and back to Lost/Stolen Phone and Resume Service this time. Log out. Restart phone.
See what happens.
2. Manually add $1 into your account and restart the phone.
Failing that then you'll need to contact the moderators. This link still works for now.