Originally Posted by normtheunsavoury
Sorry for the double post, I completely missed this!
Again, I think it would depend on what the reason was. As far as I'm aware all successor chapetrs still venerate their Primarchs, unless of course they are the Blood Ravens. As long as they can still trace their lineage back to the Ultras then I would imagine a call of enough importance would be answered.
That just leaves the question of what would the call need to be?
If it was an open call for support in taking control of the Imperium then I would imagine their would be a sizeable amount of resistance. If it was a call for aid to defend against some massive force I would think that the call would be answered and the mother of all Legions would be raised in a very short time.
Tell that to Graham McNeill. "The Chapter`s Due" sees an iron warriors warband and a daemonic warhost lay siege to ultramar and strike at several key worlds simultaneously. Utilising pirates and mercenaries as well actually.
And which chapter sends its aid?
The Raven Guard. They send not even a dozen warriors with the interest of capturing one of their own turned renegade.
Does Ultramar have to fall before their successor chapters take notice?
"...Damn. Maybe we should have helped them, eh?"