Trouble Making Calls on Macbook Air

So I have followed everything in Apple's recommended help guides. The only issue I've come up with is that after I enable WiFi calling on my phone it doesn't give me an option to add another device, but the two devices have the same Apple ID and there's nothing else I think I can do. Prior to the software updates the feature worked fine, but now I can't get it back.

I have an iPhone 5c with the most recent update running on T-Mobile's network, and my Macbook Air runs El Capitan OS. I don't have access to two WiFi networks. Any help would be highly appreciated!

*edited for typo