SIP trunks are a method for transferring voice data over the internet, similar to how data is transferred online, and while they offer more flexibility than traditional PRI ISDN lines, they typically come with specific channels that determine the number of concurrent calls possible. The number of channels can be adjusted almost in real-time, and although a SIP trunk can technically support unlimited channels, users must pay for each one. When purchasing a SIP trunk, businesses essentially buy access to a network connection rather than the connection itself, similar to a mobile data contract. Pricing generally involves a one-time fee for trunk access and ongoing charges based on the number of channels, individual number usage, and minutes used. The notion of "free" SIP trunks is primarily a marketing strategy, as the actual cost comes from usage rather than the trunk itself.