There was no call to be made! Cobb always could go home, but going home would be facing the reality of what's happened. So he ran to the dream world, and defined going back to his kids as returning to reality, heance the intensity of the seeing their faces stuff...
Cobb is lost, that's the key. His and Mal's totem were each other, hence the riddle, and so they shared their totem, a manifestation of their devotion to living their life together. But things get fucked up, as we know. What we don't know but is heavily implied is that, Cobb is lost, his totem is corrupted and model of reality is dead...maybe, or maybe she's in limbo is she was right. Cobb can't be sure, because he had no totem (or maybe he shared the top), his totem was her, and she jumped. He's of the belief that she's dead and he's in reality but there's another option, that she was right and she's in limbo, and he's not back in reality. Cobb runs because he can't come to terms with what happened because he's not even sure it did. Time and time again we get visual clues that totem aside, he's losing his grip, we see Mal images in what should be reality, and most importantly on the snow level Cobb can't shoot mal because he's unsure that she's not real. That isn't a quick emotional slip, that's him admitting in the moment that he's never been sure that she wasn't actually right, it's a key moment.
Saito knows more then he puts on, the first scene indicates a failed attempt at inception once before, without knowing who else was involved it was Saito cobb eames and Arthur, and sAito was the mark and Cobb tried to incept it. The clues are everywhere, eames and arthur have bitterness over why a last mission failed, it's cuz they just tried it, and arthur is emphatic that inception can't work, cuz it just didn't. It didn't work because it was too complicated, they tried to do it directly and it didn't incept, it was too complicated, a radical notion. But they learn from it, this time they focus on simplicity and layering, and it works.
Simply put, the first scene is the result of a failed attempt to incept Saito by eames Arthur and Cobb, and it ends in limbo. The failex experiencd gets saito though, and he figures out, and he manipulates Cobb throughout the film. He uses cobbs delusion to get him to do his bidding, but then he also helps incept cobb, he's the one who speaks emphatically twice about taking a leap of faith so they can be young together, ideas Cobb then projects back out in a 3rd time/deeper level. Thats exactly how they explain their plan re Fischer, plant it, then re plant it, then the 3rd time it will be their idea and they'll project it, Saito incepts Cobbz Saito is taking advantage of Cobb but he's also helping him get home, he's very involved in the backstory and grand workings of what's going on. He flipped a world where he was the mark and made it work towards his own goal, and helped cobb while he's at it, possibly because no one can fully escape until Cobb does if it's in fact cobbs world.
So when Cobb successfully regurgitates those ideas in limbo, Saito knows its worked when they wake up, cuz of what Cobb said. Now Cobb can go home. I don't think there was ever a call to be made, sAito knew Cobb can go home and played into cobbs delusion, knowing that if the inception took, Cobb could finally go home anyway (where as it hadnt he'd have watched that top instead of seeing his children's faces, and nothing would be fixed)
That's the genius of it all. The iffy logic things, that one call could change refugee status, why can't Cobb fly his children to Paris, they're not flaws, they don't make sense because they're not real, they're cobbs delusion. The truth was always simpler, he couldn't face his children until he let go of Mal, as long as the idea that she was still alive was in his head he could never go back until he either brought their mother back or accepts her death.
It's all cobbs journey and his being lost and incapable of knowing reality. The logistics are all just Lies Cobb believes to justify taking his time to let go of mal, or in terms of his original intent bring her back. That's why the childrens faces, always their faces, they're going to be his new pillars of reality, so he can't face them until
He knows for sure he's in reality, at risk of leaving them alone forever if he's wrong when he does do. Everything else is just convenient lies.
It's this that makes the end come together. Cobb is in the dream throughout, and he can't get out until he lets go of mal because he and mal initially built this world, so even returning to reality won't help if he's not sure it is in reality. It is at the end, where the top spins but Cobb walks away and embraces his children that miles sees Cobb is finally healed. After this miles walks off screen satisfied, and suddenly, the spinning top begins to topple. It's because miles saw Cobb was healed, and after seeing this and walked over and turned off the dream, knowing Cobb would accept reality. And it's then that it starts to topple, then cut to black.
The whole film was in Cobb and Mal's dream world, and the cut to black is it finally getting shut off. Thats why the assumption that if it's all a dream then it doesn't exist as a situation an he's not home is not actually true. The whole film is a dream, but Cobb gets back to reality, the second the movie ends Cobb is back.
