There is an article in ITBusiness.ca by Grant Buckler talking about Open Source telephony software such as Asterisk and SIPx. It includes discussion of an industry panel that had unsurprising mixed reviews. As vendors of telephony systems, some of what Open Source offers can come in direct competition with what they offer.

When it comes to supply and demand, I always believe it is best to analyze Open Source from a demand-side perspective rather than from the perspective of suppliers. Open Source is best understood as the demand-side supplying itself.

I believe there is always a need for vendors given most potential users do not have the internal skills to manage telephony, but these are vendors of support services rather than software vendors.