Voicemail!

I’ve been having issues with getting my voicemail to work properly on my last 3 iPhones I’ve owned. It started when I bought a new iPhone 8 Plus in late 2019, I was previously able to see visual voicemails but then suddenly, a few weeks after I bought that phone, it disappeared and I could only see the “call voicemail” button. I bought an iPhone 11 last year and the same thing was happening— only “call voicemail” was showing. Now I have an iPhone 12 Pro Max and when I went to check my voicemail the other day by tapping that button, it said “call cannot be completed as dialed”. So now I have to actually dial my own phone number just to access my voicemails. Not that it’s a huge deal, but I’m just curious as to what’s going on, like why do I not have visual voicemails after previously being able to use them, and why is the call voicemail button not working now? Is it a carrier issue? Do I need to do something on my end? Any advice would help. My carrier is Straight Talk.