One of the best advantages of SIP was interoperability, I worked with many vendors which were able to use Cisco/Asterisk/Skype (when they supported SIP), ACME..

One of the best advantages of SIP was interoperability, I worked with many vendors which were able to use Cisco/Asterisk/Skype (when they supported SIP), ACME...yes it was painful to troubleshoot headers and messages but most of the time worked. Many issues when protocol was initially deployed and replaced H323 but at the end it prevailed