This commit allows create call flow extension with some feature code (e.g. `*401`)
Then on the phone you can configure BLF like `flow+*401` and use `call_flow_subscribe` to
track current status of this call flow. BLF like `*401` will also works but only if
use `call_flow_monitor` script.
Also it is possible dial `flow+*401` as well as `*401` to toggle call flow state.