Under the FTC's 900-Number Rule (16 CFR 308.5) every pay-per-call message opens with a free preamble, slow and clear, in the language of the program: the provider's name and what the service is; the cost - the flat fee, or the rate per minute and any minimum; that charges begin after a tone and the caller can hang up within three seconds of it without charge; and that anyone under 18 needs a parent's or guardian's permission. A call that cannot cost more than $2 needs none. A key-press to accept the charge can stand in for the tone. Frequent callers may skip the preamble - but not for 30 days after a price change.

Every ad for a 900 number states the cost clearly and close to the number, carries the notice that callers under 18 need a parent's permission, and is never aimed at children under 12.

A caller is never billed more than the preamble stated, never for a call on a line whose preamble falls short, and nothing at all if they hung up within three seconds of the tone. Time-based charges stop the moment the caller hangs up. Enter each call's length and what was billed; the bill is checked against the line's own price.