Why don't the desk phones ring when someone calls?

Since roughly December, our company has started experiencing an issue more and more frequently (now something we deal with multiple times per day) where our desk phones do not ring when they are called. This happens at random across multiple extensions at two different offices. We've got plenty of bandwidth, our network should be more than enough to support our 25 phones. We've made repeated attempts to work with Ring Central on this issue and they continue to brush us off for days at a time. Has anyone else dealt with an issue like this?