Visual Voicemails Delayed

I have an iPhone 6s. VERY frequently my visual voicemails are delayed by hours or days. In many cases they don't get delivered until I power down the phone. I suspect that this may be happening if I am connected to WI-FI when the call comes in (I have WI-FI calling enabled).

T-Mobile is my carrier. I am running iOS 9.0.2

HELP