Session end and capacity pools

A trunk allows a fixed number of concurrent calls, shared by inbound and outbound. That limit is declared once as an engine capacity pool and referenced from the channels:

sutra:
  capacity-pools:
    trunk-main:
      slots: 30
      reserve: { inbound: 6 }      # outbound can never take the last 6

  - name: call-start
    transport: voip
    bind: "number:+15551230000"
    pool: trunk-main
    pool-call-key: header.x-call-id    # one slot per call
    session-end:
      on-message: call-hangup          # the hang-up frees the line; the instance runs on

• A call to the number takes a slot or is refused busy.
• An originate waits in the outbox, oldest first with no attempt counted, until a line frees, so a batch of calls paces itself to the trunk.
• The slot belongs to the call, not the instance. session-end returns it at the hang-up even when the instance goes on with post-call work, and an end or failure returns anything still held.

The pool settings are engine configuration: see the configuration reference in the Sutra book.