Q: What is Wifi-calling and how does it work?
A: Wifi-Calling is the feature that has been introduced with the latest iPhone and android variants. Currently T-Mobile & Sprint in US offers this service and a couple of other service providers are expected to start the service.
The operation of wifi-calling service is different from the rest of the VOIP services. The service initiates a DNS query to locate its packet data gateway. It establishes a persistent IPsec connection with the gateway. Traffic from all the services and applications offered by the service provider is carried over the IPsec tunnel. These services include but not limited to SMS, MMS, call-signaling, wifi-calling etc.
The operation of the Encrypted Audio Heuristics feature for wifi-calling consists of the following parts:
DNS-based Gateway Learning
Heuristics-based Media Classification
Note: This feature is introduced in 6.5 and not before that.