iPhone users who are communicating over an unencrypted network will receive a warning and the option to end the call immediately. The unsecured call call warning seems like a direct response to presentations like Practical Cellphone Spying at DEFCON that demonstrated the ability of attackers to set up a phony GSM base station and intercept cellphone calls. The warning is new with iOS 5, and users will be able to end calls over unsecured networks.
Source: https://threatpost.com/unsecured-calls-5-0/91869/