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A network timeout tells you nothing about whether the request arrived. Without a way to say “this is the same request”, your only options are retrying and risking a double charge, or not retrying and losing the work. Send an Idempotency-Key — any unique string up to 255 characters, a UUID by convention:

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Where it is required

POST /v1/calls and POST /v1/billing/topups — the two paths that spend money. Omitting it returns 400 missing_idempotency_key. Other mutating endpoints honour the header if you send it.

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What happens on a retry

Replaying the same key with the same body returns the original response — same status, same body — with an extra header:
Keys are retained for 24 hours. After that the same key is a new request.

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Two ways it can refuse

Generate a new key per logical operation, not per attempt. Retrying a failed call with a fresh key is a second call. Retrying it with the same key is the same call.