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 480 Hz and 620 Hz at a cadence of 0.25 seconds on, 0.25 off; that is, 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 both sound the same, only that the busy signal's beeping sounds have a slower cadence than the reorder tone.