Michael,
As I can understand by your description, you want a queing mechanism to queue call messages to back office, they are currently sent using the web services, so there is no queuing.
BizTalk comes with two queuing adapters out-of-the-box, namely MSMQ and WebSphere MQ (MQ Series), so you can use one of these two no problem, if filenet supports them, on the other side of it you can make a web service call from BizTalk to your back office. The queing inside biztalk is database (Sql Server) driven as far as I know, so it is not the traditional queing system. So you need to decide on first how do you reach BizTalk first, would you prefer to use a queue to reach BizTalk or some other mechanism like SOAP, File etc. then once your messages is inside biztalk, it gives you the safety of database driven queing.
By putting BizTalk in b/w you'd get the benefit of assured delivery of messages even if the back office system is down, Centrallized management of all the calls etc.
Regards,
Sajid