In DTMF selective calling, the radio is alerted by a string of digits. Systems typically use 2- to 7-digits. These can be dialed from a traditional telephone dial connected to a radio or may be generated as a string of DTMF digits by an automatic encoder. In some systems, a dispatching computer is connected to a DTMF encoder via a serial (RS-232) cable: the computer sends commands to the encoder that generates a pre-defined digit string that is then sent to the transmitter. Dgits are usually sent at a minimum of 55 milliseconds (ms) in length with at least 55 ms of silence between each digit. Some decoders may require much longer-duration digits. DTMF digits consist of paired tones: a row tone and a column tone. The levels of row and column tones must be similar in order for a decoder to interpret them reliably. Radios with DTMF decoders may monitor all system traffic or remain muted until called, depending on the system design. When the radio receives the correct digit string, it may momentarily buzz or sound a Sonalert. An indicator light may turn on and remain latched on. In most systems, the radio's receive audio would latch on after receiving a valid digit string if normally muted. Many companies have trademarked names for their DTMF features. For example, Motorola calls their DTMF options, Touch Call. Because DTMF is a standardized format, most of the features are interchangeable. Generally, any radio that is equipped to decode the digit string 0-1-2-3 would be compatible with any system using DTMF. Some systems use DTMF for push-to-talk unit ID. Each time the push-to-talk is pressed, the radio sends a string of DTMF digits. Each radio has a unique string of digits. This allows the base station to know who last called or who last pressed the push-to-talk.