This solution will take you through how to delete a number.

Via the client's 'Numbers' tab, observe any numbers that you wish to remove that are attached to 'SIP account' (trunk) and if any are, right-click and select 'Release from trunk'.  It is only when a number is not assigned to a trunk that it can be deleted / moved to another client account.

Whilst still on the client's 'Numbers' tab, select the number, right-click and select 'Delete from system'.  Note that the number can no longer be re-obtained once this action has been done.
If instead you wish to retain the number either leave it in place or move it to another account.
For more information on assigning a number to a different client account, please refer to our solution, Number Properties, in our solutions area.