Hopefully soon. It's enabled on my phone, and I installed iOS 9.0.1 on my wife's phone last night to enable WiFi calling while we're on vacation. Unfortunately, I can't activate her WiFi calling because she wasn't in the beta program.
The FCC has not provided a time for their approval process to complete. The issue is related to hearing impaired communication technologies. The service is ready but not allowed for commercial use until the waiver is signed.
My reading suggests it will only show up on iPhone 6 and newer. (6, 6+, 6S, 6S+). I don't know for sure but I have not seen anything that suggests it will go to older devices though many are out there and could benefit from the service.