Pactolus
Where it's based: Marlborough, Massachusetts
When it was founded: 1999
Website: www.pactolus.com [1]
Why it's Fierce: Pactolus doesn't grab a lot of headlines, but they got on the open-source and SIP bandwagon way early, founding SIPdev.org in the process. Today, the company's SIP/IMS-based apps process 3 to 4 billion minutes of usage monthly in over 140 service provider networks worldwide.
If you want a service creation environment for SIP, there's RapidFLEX running on industry-standard Linux or Solaris hardware with a supporting XML-based language, XTML (eXtensible Telephony Markup Language). Don't want to build apps from scratch? There's a portfolio of pre-built SIPWare customizable turnkey services to tap into, including residential/business Class 5 VoIP, large operator-assisted event and reservation-less audio conferencing, prepaid/post paid services and voice messaging.
Links:
[1] http://www.pactolus.com
[2] http://www.fierceenterprisecommunications.com/special-reports/onrelay-top-voip-company-2008-fiercevoip-fierce-15
[3] http://www.fierceenterprisecommunications.com/special-reports/fiercevoips-fierce-15
[4] http://www.fierceenterprisecommunications.com/special-reports/simplesignal-top-voip-company-2008-fiercevoip-fierce-15