Phone Beeping Every 15 Minutes or so

The desk phone of one of our users is beeping every 15 minutes or so. The phone is a Poly VVX250. I’ve replace the phone and patch cables, moved it to a different switch port, and rebooted the switch, but nothing seem sto help. I also set up another VVX250 in my office with the user’s extension and it too is beeping about every 15 minutes. So, it seems to be an issue with the user’s account or line. AI support was useless and my support plan doesn’t allow me to talk to humans, so I’m hoping someone here can help.