A “trunk” is a single transmission channel between two communication entities; each enity being either the switching center or another node.
The “trunk” allows other circuits to be shared using well known multiplexing techniques. In the UK a “trunk call” means a call between different
towns and cities. The term is the left overs from the analog era. Compare this with “director” or “non-director” calls.
For your case a “trunk” will be a ISUP TX circuit that will be controlled by Common Channel Signalling that is known as C7 or ss7.