Your tag line says that you are running iOS 18.
I am running iOS 18, and my iPhone does not convert voicemail to text messages (notifications). It does transcribe voicemail Into text that I can see if I go into the Phone application to look at voicemail - but I do not remember ever having seen that text appear, unsolicited, as a text message. (Then again, I am not a T-Mobile customer.)
Google search results suggest that this is a carrier-specific feature, and Google AI results even mention a couple of T-Mobile-specific settings to try changing (one in T-Mobile's Visual Voicemail App, one in your T-Mobile account on the Web) before calling T-Mobile. However, I have no idea whether these AI results are accurate – or if they merely "sound good", so I'll go with the answer in this Apple Commuities thread.
How to turn off T-Mobile Voicemail to Text on my iPhone 12 – Apple Community
"You need to speak to T-Mobile about this. Turning voicemail into a text message is not an iPhone feature. That is something configured by T-Mobile."