This varies per provider but for the most part, you can consider the SIP Trunk a PRI (don't mistake for a PRI this is just for example purposes). You purchase a trunk\PRI. You purchase how ever many lines\DID's, etc. Instead of being limited to 23 paths on the PRI you purchase the amount of channels\paths on a SIP trunk and that determines how many calls you can have at the same time. You really have to discuss this with your provider(s). Some charge more than others and some bundle to save you money. Each path\call on the trunk is about 80kb. Most like to charge you by paths. This is a condensed explanation. It's not that simple but may help.