I dont know alot about servers and much of how they work but I believe the way this game is set up...you need to be on the same cluster as your friend.
Easy way to tell if you are is, stand around an area for a few moments and if your friend is on the same server as you...you will see a phantom of his character for 5 or 6 seconds(it slightly delayed, shows you what he/she was doing a few moments ago)
IF you can see them, you can summon them. Otherwise you are not on the same server and I am fairly sure there would be no way to summon at that point as the signs and msgs cycle around that cluster, not the entire game and all of its servers.
Game I am looking forward to the most? No Man's Sky.