Continuity - Can't answer iPhone calls on Mac/iPad

Mac - El Capitan 10.11.6

iPad - iOS 9.3

I've got all the devices set up to be able to answer calls on the Mac & iPad according to this:

https://support.apple.com/en-us/HT204681

But it doesn't work for inbound calls. I can make outbound calls just fine. I do have a Panasonic DECT system hooked up to the iPhone could that be interfering with inbound calls being handed-off? It just seems strange that if outbound calls work, inbound should as well. Text message handoffs work perfectly.