It’s fairly easy and well document how BizTalk can call a webservice (either .asmx or WCF).
It sounds like your company chose BizTalk as a middle ware to avoid writing point-to-point systems.
In other words, if system A needs to call System B, instead, A calls BizTalk (let’s call it “Z” and BizTalk “Z” calls B). BizTalk orchestrations can also be published as either .asmx or WCF web services. So Application A can call BizTalk as a web service. Then when System B changes, system A hopefully doesn’t have to change, because you have the “Z” layer in between.
I’ve also seen companies use BizTalk to aggregate several web services. Example, System A needs to call System B and System C. So you write an orchestration that calls System B and C, and combine the results into one schema, then publish that as a web service. System A then calls the BizTalk webservice.
Hope that is kind of what you were asking.
Neal Walters