In the days of old there was no such thing as the Internet, cell phones or the like. When a noble man or clan cheif needed the clans to gather there are many ways this would be accomplished.
One way that the clans would be called to war is the bagpipes, the bagpipes ave a very distinctive sound that carries and bounces. Listen to a bagpiper in a close area or where there are metal buildings, the sound bounces and echoes something fierce. The call would go out to the bagpiper who would start playing a distinctive tune, the sound would carry on to the next piper, who would start playing as well and so on and so forth.
Another way that the clans would be called was by fire, runners would be dispatched carrying a large piece of wood in his hand. Attached to the wood was a horizontal piece that was alight with fire and smoking.