If you think adhering to standards would straighten out kinks in deploying SIP-based telephony systems, think again. According to this article posted on SearchUnifiedCommunications.com, standards-compliant SIP products don't necessarily equate to pain-free interoperability across different vendors.

Partly to blame is the number of features standardized by SIP, which isn't that many. As a result, vendors are left to create their own extensions to SIP, which in turn makes their technology proprietary. Read Article