A call is a process instance

One call is one instance of one BPMN process. It starts when the call-start channel, bound to a number (bind: "number:+15551230000"), receives the call, and it ends when the process reaches an end event, which may be well after the hang-up if there is post-call work to do.

The instance holds the conversation in its variables. Every message of the call carries the call’s id in the x-call-id header (plus x-caller, x-called and x-call-direction), and the start event declares an alias over that header so that later messages find their instance:

<q:alias name="callKey" expression="getValue(header, &quot;x-call-id&quot;)"
         unique="true" onConflict="correlate"/>

Waits

Everything the call does between steps is a wait: for the caller’s next turn, for a key, for the hang-up, for an idle timer. A conversation is therefore a loop that returns to the same event-based gateway after every answer, and each pass is a new wait with its own timer and correlation. Give every wait of the call an arm for call-hangup: a message for a wait the instance is not parked at is refused.

Why this matters

• A retry rule is a loop with a counter in the model, visible in the diagram, not a branch in a bot’s code.
• The same engine features apply to a call as to any process: audit, personal-data tagging and erasure, redaction of sensitive variables, deployment packages, hot reload.
• Post-call work (analysis, a case record, a follow-up) continues in the same instance with the call’s data to hand.

See Calls and the media protocol for the message sequence.