Start a subscription checkout

Returns an embedded Stripe Checkout clientSecret to subscribe the account for the given interval. Quantity is set server-side to the account's current phone-number count (one unit per number) — Dial doesn't subscribe a subset. Coverage begins when the invoice is paid. Not available during impersonation.

Accounts holding an iMessage number can't subscribe — iMessage is pay-as-you-go only, so the request is rejected with 403. Release the iMessage number first.