Here is a good Whitepaper on SIP, as well as a cost analysis our partner Curt Island with TelVersant Group did for a customer. The spreadsheet compares traditional LD usage with PRI and SIP ,which shows that from a cost savings perspective the difference is negligible. Curt tells us that net/net – if a customer is looking for cost savings then they can achieve that with traditional PRI’s.
However, if a customer has an end goal of going all IP, then a SIP application has its place.
Also, click here to read an article from XO: “Why SIP Makes Sense”.