Added session duration to sip_audio_session.py

darcs-hash:20081205175120-58103-536b1a0a177c4cd60bb8c9707e864750eb105a52
Remove "-" if the target argument of sip_audio_session.py is obviously a telephone number

darcs-hash:20081205175936-58103-56d83cea9524ad9c88cb910a44530738be172f0b