Phone numbers to dial (1–1000 per request). Each item is either a plain phone number string (legacy format) or an object { phone, call_context } used to attach free-text, per-call context. Note: a large batch (up to 1000 phones) where most items carry a near-maximum call_context (1500 characters) can push the JSON payload past the server's 250kb request body limit, which returns 413 Payload Too Large before validation even runs — this is a practical limitation to keep in mind, not a bug.