This solution will take you through step by step on how to merge two clients/trunks. If Trunk A for Client A is the trunk that will remain, and Trunk B for Client B is the trunk that will be merged into Trunk A:
1. via the 'Numbers' tab for Client B, un-assign any numbers from Trunk B (except the headline number, which cannot be unassigned)
2. via the 'SIP Accounts' tab for Client B, delete Trunk B
3. via the 'Numbers' tab for Client B, select the numbers that were assigned to Trunk B and select 'Assign to owner', selecting the Client A
4. via the 'Numbers' tab for Client A, assign the numbers that were allocated to Client B to Trunk A
5. via the 'Options' page for Client B, press 'Delete' if Client B is no longer needed
Ensure that you have configured your PBX appropriately now that Trunk B no longer exists, and as Trunk A now has all numbers assigned to it.
Note that calls will not be possible to the numbers assigned to Trunk B until all steps above have been completed.