Phantom voicemails with voicemail off

[Futaura]

Ever since I moved to O2, and ported my existing number in, I've been regularly getting weird voicemails. The odd thing is that voicemail is off, I have set a PIN, am now using Call Alert, but I am still getting these phantom voicemails even when my phone is on!

The content of these messages ranges from apparently random electronic noise, sometimes up to 4 minutes long, to automated messages such as "you are connected to the audio test interface" and "the person you are calling is unable to take the call".

Any ideas???

[MI5]

Really weird that one!
Never heard of anything like it before but I suspect you’ll need to call someone in customer service to escalate the issue to the network team.

[jonsie]

You don't have cell broadcast turned on in Messaging?

[Futaura]

Not as far as I know. Can't see any relevant settings for broadcasting in Messaging/Messages. Am using a Sony Xperia XA1 Dual SIM running Android Oreo.