Consider the following ED workflow:

Start Call intercept -> do a lot of "Call REST Service" and "Read from DB" -> Call Allow / Drop / Transfer

Is it mandatory to use the SuspendCall step as the first step in my Call Intercept workflow when starting the flow?

Perhaps related: In the Avaya Breeze Snap-in Development Guide, Chapter 6 "Performance and scalability considerations", the call.suspend step is also mentioned to enhance performance:
- Important: Be sure to invoke the suspend method as shown above (call.suspend() ) before invoking the asynchronous method.

suspend releases sip related resources, so that workflow can do some thing in background (like REST in your case), and then make decision on call and then sip would come back into picture

so yes suspend is useful in releasing sip threads

Ok thanks.

I will add the SuspendCall step at the beginning of the workflow.