Sounds like the T-Mobile voicemail paging service.
Here's what the support document has to say (link below):
Receiving messages from 129
129 is the T-Mobile identifier for the voicemail paging option. When a caller selects the option to leave you a numeric page, the device receives a text message from sender '129' containing the phone number of the caller.
- Call voicemail.
- Turn off the voicemail pager notification by following the menus. See VoiceMail Menu & Shortcuts.
- If you still have the issue, contact us.