Dedicated (non-shared) channels are assigned to a specific DID, and allow additional simultaneous calls only on that particular number. Note, that by default, all DIDs are provided with two dedicated (non-shared) channels at no extra cost. But you may increase the concurrent call capacity of your selected numbers by adding extra channels. An additional dedicated channel cost $20 USD per month + a one time setup of of $20USD.
Flexible (shared) channels cover a group of selected DIDs, and allow additional call capacity on all those numbers. As additional incoming calls are received, flexible channels are dynamically allocated to DIDs within a specified group on a first-come, first-served basis. The group of numbers, flexible (shared) channels are assigned to, may be comprised of DIDs from anywhere in the world, for example, DIDs from Canada, Spain and Australia may be included in the same flexible channel group. An additional flexible channel cost $20 USD per month + a one time setup of of $20USD.
It should be noted that, by default, all DIDs are provided with two dedicated channels, over and above any flexible channels purchased. This ensures that even if all flexible channels allocated to a group are occupied, your DIDs have the capacity to receive (at least) two concurrent incoming calls.