Trunk circuits are either setup as ingress, egress or a combination, i.e.ingress and egress. As an example for cross border circuits I have used specific ingress and egress circuits with configured policing masks.
For your case you have to consider the total offered traffic. This will mean that you will need 15 Erlangs.
To dimension a E1.0, etc.. circuit you will need to look at the total offered traffic. This will help to determine if you will need additional circuits.
VBR/ Wallis Dudhnath