I'm with Vodafone UK and WiFi Calling is working here fine.
Which model are you using and on which carrier?
I'm currently on T-Mobile US iPhone 6s...mine is just stucked on waiting for activation.
Not with wifi calling, but after changing my SIM card, I cannot activate iMessage or FaceTime on my iPhone 6s Plus on AT&T. Running iOS 10.0b2.
I downgraded back to iOS 9 and my wifi calling started working again.
I've got an iPhone 6 on iOS 10 public beta and at first my T-Mobile Wifi did not work but after I turned it off and back on in settings and shutdown / restart it kicked back on. Try that and see if the works.