I can't check my voicemail on my Apple iPhone 13 Pro iOS 15.0
Cause 3 of 3: You're trying to check your voicemail when abroad
The default voicemail number can only be used if you're calling form your home network.
Solution: Call 172 to check your voicemail. If you can't call 172, call +353 (prefix) 5 (your phone number) instead to check your voice messages when abroad. E.g.: +353 87 5 1234567.