trying to shut off Voicemail on Iphone 14 using O2 in Europe

We got an Iphone 14 in the U.S. unlocked and brought it to the Czech Republic. I have had an older Iphone on the same carrier, O2, for several years. I have never had Voicemail, never set one up. It is very unusual to have a Voicemail in the Czech Republic.

A few weeks ago, a Voicemail appeared on my phone to callers. It answers after only a few rings and I want to shut it off completely. I have spent hours on the phone with the phone carrier, as well as used several of these codes you see in YouTube videos for shutting off Voicemail.

Nothing works. The phone carrier says it must be the device. I have no idea how to shut it off and have tried everything on all searches in this community and using google search to shut off Voicemail on Iphone.