I was having the same problem on an Iphone 12. I set up the Visible service and transferred from Verizon and moved my phone number over. Calling/receiving call, email, texts and data worked great. Voicemail not so much. I never got the "set up voicemail" prompt button on the voicemail icon in phone app. The only thing I could see was the old voicemails from Verizon. I spent at least 3 hours with Visible via chat three different times. They can reset your voicemail but that didn't work. They couldn't figure it out so they referred me to contacted Apple support. Apple support researched it and stated it was the carriers’ issue. At this point I was going to give up and find another carrier. I decided to try one last thing. I decided to delete all the old voicemails from Verizon and reboot the phone. Walla! it just started to working! I never had to "set up my voicemail" it just worked.
Bottom line:
Make sure the sim card is out of the phone. Assuming you are using the Esim.
If you get the setup voicemail prompt, congratulations you have it easy, just follow the instructions.
If you don't get the setup voicemail make sure you delete all voice mail and I do mean to delete them all! Even the retrieve old voicemails. Don't forget to reboot the phone afterwards.
I'm very surprised that Visible and Apple did not know this one easy possible solution.
