Flexible (shared) channels cover a group of selected DIDs, and allow additional call capacity on all of these numbers, notwithstanding their different area or country codes, e.g. DIDs from Canada, Spain and Australia may well be included in the same group (pool).
DIDs within this group will be dynamically allocated with necessary additional channels on a first-come, first-served basis.
Please also note that by default each number provided by Virtual Phone Numbers is equipped with 2 voice channels; therefore, additional channels are necessary only if you wish to further increase the default capacity of your numbers. An additional flexible channel cost $20 USD per month + a one time setup of of $20USD
The 2 default channels also serve as a guarantee that even if all flexible channels allocated to the group (pool) are occupied, each of your DIDs will still have the capacity to receive (at least) two concurrent incoming calls.
For information on dedicated channels please refer to: What is a Dedicated (Non-shared) Channel?