A grey route is a route by which traffic enters a telecommunications network that is not sanctioned by the mobile network operator (MNO). Typically, this will mean international A2P messages being sent as though they were cheaper traffic such as local A2P or P2P.
Sometimes grey routes can refer to routing practices which are not illegal in the sender’s country but are illegal in the receiver’s (the victim of the grey route in this case). This makes it very important that MNOs not only consider their firewall setup, but also have a clear view of the legal and commercial aspects of their business.