+14155559876.
from_number_id can be any phone number on your account. It does not need to be assigned to an agent, and any agent can call from it.
To place a call from the Playground, open the agent, click the phone icon in the header, and under Outbound calls select a From number, enter the To number, and click Place call. The call uses the agent’s published version.

Place a call from the agent header
Response
200 when the request is accepted. The calls array has one entry per destination, in the same order. Each entry carries either an agent_call_id or an error, so check every entry rather than the response status alone. Use agent_call_id with Get Call to follow the call.
ringing_timeout_seconds sets how long each destination rings before the attempt times out, from 5 to 80 seconds. The default is 60.
To personalize a managed agent, set outbound_calls[].dynamic_variables with values such as { "customer_name": "Jordan" }. See Dynamic variables for examples.
Cartesia numbers place at most one outbound call per second. Faster requests are queued.