SIP Trunks (Session Internet Protocol) were the most popular thing since sliced bread 5 years ago.  They allowed for data vendors to get involved in a the voice side of telecom.  This was a sector up until now that they weren"t able to assist their clients in.  In terms of functionality SIP trunks were/are a practical avenue to achieve VOIP standards.  They allow for the end user to achieve the advantages of PRI (namely having multiple DID"s per trunk), while not requiring the end user to have to pay for the local loop.

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The problem occured and still is in some cases with the lack of a standard for SIP trunks from the providers.  Consequently, every provider had a short list of compatable systems.  Over time these lists have expanded, and there are several providers including Windstream that  now allow for any IP PBX to be compatable with their SIP trunk offering.  However, this may have been too little too late.  Most providers instead of working with the data vendors decided to come out with a more turn-key solution.  In this solution thay used the same SIP technology, but instead of allowing a middle man to provide the phone equipment and hardware and the carrier provides the entire solution.

From a customer perspective, the turn-key solution works best.  It elimates teh finger pointing when an issue arises, as well with most carriers there is limited to no capital expenditure on equipment and maintenance.  Therefore, I believe SIP trunks are a viable option to replace POTS and ultimately PRI.  The beauty now is that the control lies with the customer as to whether they want to work with a carrier directly or own the equipment and just purchase the network services from the carrier.