This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
I think that what should be called from a sequence diagram is not another sequence diagram but a use case. That is because these sequence diagrams model use cases. It is UML compliant to call an use case from a sequence diagram. Hope this helps, Adrian