Problems with iOS app ringing consistently

We have dozens of users using the iOS app and we have found that it doesn't ring the app consistently. Some users with deskphones report that sometimes it rings their app as well and sometimes it does not. We know the users are setup correctly because at times it will work correctly. I have seen on my app that it will never ring, but immediately popup a message that a call was missed. I don't think reinstalling the app would help because it is affecting so many users. We have a mixture of carriers (Verizon and AT&T) and the problem isn't isolated to when the device is on Wifi or on cellular network. One user actually has a iPad and iPhone that both have the app and they informed me that sometimes it rings one and not the other. In some cases it will ring them simultaneously as we would expect. So it is an intermittent problem.