iPhone 5C voicemail issues

We recently switched from ATT to US Cellular. We ported over two numbers from iPhone 4 to iPhone 5Cs. One did not have visual voicemail at all at first. After talking to 4-5 different people through US Cellular retail store and customer support line over the span of 3 days and too many hours it did finally come up, but it's sporadic. I still do not get notifications of when I have a new voicemail consistently. It's mainly me calling in to check by holding the one and then 3-4 will show up days later. I'm still only about 10 days into this and wondering if we've made a big mistake switching to US Cellular and I just need to take the phones back and try another carrier or is there a fix for this very frustrating, what should seemingly be easy to fix issue? We had iPhone 4s with ATT for over 3 years and data on or off we got voicemail notifications. Data off voicemail notification to call in, a red circle with the number would pop up or data on still a notification just would show up in visual voicemail instead, so what is the fix? US Cellular is pretty much saying it's Apple's fault and from digging around on the internet help boards it seems like a lot of finger pointing back and forth, which is great, but I really need this issue resolved because I still live in the Stone Age where people actually call, talk, and leave voicemails and all of my communication isn't via text, so what's the fix? It's a hassle (I know 1st world problem here) to just call into voicemail after every missed call, but I shouldn't have to do that when our old phones worked fine. And again why are visual voicemails coming in days late if at all? Thanks in advance for any help and just to clarify I've had them reset voicemail box, reset network settings, they had to fix something in their system to get the visual voicemail to even come up, so I'm not a total dummy when it comes to iPhones (normally) I can usually troubleshoot on my own and figure out more than the tech guy at the retail store, but I've taken this everywhere else I know to ask so a fix would be great. Keeping in mind I've done the basics and taken it through 4-5 of their tech assistance people to troubleshoot.