Unless you have been living in a cave the last few years, you know that hundreds of service providers have popped up with the plan to sell telephony services to enterprises via SIP. SIP Trunking offers the promise of eliminating the separate TDM (T1/E1 or Analog) trunking to the local telephone company, replacing it with services over existing or dedicated broadband internet.
However, IMHO the promise is far from being delivered upon and there are a range of issues that we in the industry must solve before charging ahead. What challenges? See the article I just had published in Internet Telephony : Feature Articles - SIP Trunking: What Challenges Lie Ahead?