IP-PBX                                         P-CSCF
    |                                               |
    |----(1) INVITE-------------------------------->| 
    |        Proxy-Require: mediasec                |
    |        Require: mediasec                      |
    |        Security-Verify: msrp-tls;mediasec     | 
    |        Security-Verify: sdes-srtp;mediasec    | 
    |        Security-Verify: dtls-srtp;mediasec    |
    |        (SDP) a=3ge2ae:requested               |
    |        (SDP) a=crypto:1 AES_CM_128_HMAC_SHA1_80 inline:EpcgtOdT5qd...
    |                                               | 
    |<---(2) 100 TRYING-----------------------------|
    |                                               | 
    |<---(3) 180 RINGING----------------------------|
    |                                               | 
    |<---(4) 200 OK---------------------------------|
    |        (SDP) a=crypto:1 AES_CM_128_HMAC_SHA1_80 inline:lnfakjh2sd1...
    |                                               |