Ring Central App doesn't ring

Hello, my Ring Central App doesn't ring at all. I just get a message pop up informing me of a missed call and voicemail is usually left. Sometime my phone lights up as if I touched a button then goes dark again, but nothing on my screen at a to show me it's ringing until it's too late.I use an i Phone iOS 10.3.2. it's a US account but I use mine in the UK when i'm on good/strong wifi. It did used to ring when I first had it but stopped a few months ago. I'm hoping you can help.