When deploying SIP trunking, it is not necessary to purchase a SIP trunk channel for every extension, rather, only enough to accommodate simultaneous calls. For example, if an enterprise has 100 telephone extensions, but at any given time only 25 people are on the phone, then only 25 SIP trunks are necessary to accommodate normal traffic. Some oversubscription to that figure would be a wise investment, but even with oversubscription, SIP trunking affords a much more efficient use of resources.

Scalability is also an issue with any telecommunications environment, and SIP trunking allows for easy additions. When additional extensions or higher phone traffic demand more SIP trunks, they can be purchased incrementally, one at a time, on an as-needed basis. A PRI solution on the other hand, has a 23-channel incremental purchase requirement, which may mean that businesses would find themselves buying more channels than is really needed. Another comparative advantage between SIP trunking and a PRI solution is implementation time; a provider may require as much as 30 days to provision PRI channels, and it requires an on-premises visit; whereas SIP trunking can usually be rolled out remotely within 24 hours with no truck roll required.