SIP softphone buffer overflow demo
Publish date: Aug 8, 2007
Someone was showing off a 0day exploit at Black Hat. The article is a bit sketchy and feels sensational, but it does show that various parties are concerned. Just like most other pieces of software, softphones will (and do) have security vulnerabilities lead to remote access.
Article can be found here.
Hardphones, on the other hand, are secure.. right? :-p