So.... you know why Sollecito was called in. Just because they scatter-gunned Sollecito with questions meant to trip him up, does not establish that that was the reason they called him in.
Besides... IF they called him in over the "phones turned off" thing, please reread Grinder. What is "normal" for people who only knew each other a week?
The fact is that the cops already had their theory BEFORE calling Sollecito in - one cop said he was sure the Knox was also summoned. At interrogation it is not the goal, anyway, to establish fact. The goal of every interrogation everywhere is to get those being interrogated to confessed to the already established set of facts - established in the cops' minds.
"They buckled, and told us what we already knew."