The reorder tone, also known as the fast busy tone, or the congestion tone, or all trunks busy (ATB) tone of a public switched telephone network (PSTN). It varies from country to country; in North America it is a dual-frequency tone of 620 Hz and 480 Hz (roughly a slightly low D-sharp over a noticeably low B-natural on an equal-temperedA440 scale, i.e., an interval very close to an equal-tempered major third and fairly close to but noticeably wide of a justmajor third) at a cadence of 0.25 seconds on, 0.25 off, i.e., two beeps per second. It is used to indicate that that all circuits (trunks) are busy and/or the call is unroutable, or sometimes that an invalid code has been dialed. A PBX will often also do this for an invalid extension, while dialing an invalid telephone number on the PSTN usually results in playing the triple special information tone and a recorded announcement. Many phone companies in North America often play the reorder tone after the second reading of a recorded announcement explaining why the call has failed. Reorder tone is sometimes confused with the busy signal because they use the same frequencies and differ only in that the busy signal's beeping sounds have a slower cadence than those of the reorder tone.