An elite student who is unsociable and hates forming relationships with others, Squall aspires to walk the path of solitude like a lion. Whether out of personal charm that attracts others, or just a coincidence, a lively crowd is often seen by his side.
A cold, taciturn youth who wields a gunblade, a weapon part sword, part gun.—Dissidia Final Fantasy description
Squall is Balamb Garden’s most skilled mercenary. Yet, he’s cold, often to people who want to get to know him. He has a habit of putting off others with his curt speech. On World B, he’s much like how he is in Final Fantasy VIII: cold and standoffish:
Warrior of Light: “You are strong. Don’t you have any intention of using that strength to protect the others?”
Squall: “Fighting together’s not the only way.
So when did Cosmo call Squall to World B? It could possibly be at any time he loses consciousness.
During a mission to Timber, Squall passes out on a train. He then has a dream about Laguna. However, Cosmo’s could have called Squall anytime before and after the dream, before regaining consciousness. This is Ellone’s doing; she sends Squall’s consciousness back in time to inhabit Laguna. During this time, it’s possible that his consciousness transports to World B, a place outside of normal SpaceTime. He’ll have several dreams with Laguna much later in the game, with several opportunities of falling asleep.
Another scenario is his untimely “death”. Edea’s stabs him during her parade with an Ice Strike. Afterward, he loses consciousness and falls from a float. When he awakes, he realizes that he has no wounds. How? Perhaps he healed in World B?
However, he’s a bit more open to the idea of relying on friends. Sort of…
Squall: “I have only my own strength to rely on. That’s enough. Just you wait, Ultimecia.”
Ultimecia: “So you’ve come– having abandoned your comrades only to satisfy your own desires.”
Squall: “I only came to fulfill my promise. It’s because of them that I can’t allow myself to stop. I know that I’m not alone.”
Squall: “That’s why I can fight by myself!”
Ultimecia: “Interesting. Then fight you shall.”
In fact, it seems like most of Squall’s character development happens in World B. If so, then he might have happened across World B later on in Final Fantasy VIII.
A final scenario is during the Void-like area in Time Compression. Before Rinoa appears to find him, he might have traveled into World B.