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Why?
Purposeful
Traditional PBXs require phones to be made by same vendor of PBX
Translated into similar behavior for IP PBX vendors
Poor Implementations
Bugs
Incomplete Specs
SIP does too much and not enough
Too many ways to implement a feature
Different approach selected by UA than servers
Different approach selected by different UAs
Specific capabilities are missing
Line indications in SLA
PPT Version