Found Michael's post on RequestHeaders (
http://blog.greenl.ee/2011/10/07/man...-headers-ucma/) and it is exactly what I needed.
Code:
SignalingHeader subnetHeader = args.RequestData.SignalingHeaders.LastOrDefault(h => h.Name == "ms-subnet");
string CallerSubnet = subnetHeader.GetValue();
gave me all I wished for.
Thanks