am 2c457326: am 421c34c1: SipPhone: revise hangup() in SipCall and SipConnection.

Merge commit '2c4573263a97cc80f441824a75d6502ae04292a8'

* commit '2c4573263a97cc80f441824a75d6502ae04292a8':
  SipPhone: revise hangup() in SipCall and SipConnection.