Probably you don't need this file at all, default values are good enough. Actually calback.config is undocumented file. Mgetty sources has an example: # # this is a very much undocumented sample file for "callback.config". # # it's only good to show what options exist. See the source for docs! # dialout-devices tty5a:tty5d retry-time 30 max-time 90 debug 9 dial-prefix ATX3D Here is example from my working Linux box: # # this is a very much undocumented sample file for "callback.config". # # it's only good to show what options exist. See the source for docs! # # dialout-devices tty5a:tty5d delay 10 retry-time 30 max-time 90 speed 115200 modem-init "" ATQ0V1H0W1+MS=11,1,2400,33600 OK AT+FCLASS=0 OK debug 4 # dial-prefix ATX3D I looked to callback sources and found there possible options with default valuses: dialout-devices - ttySx to dial out. delay - min. dealy before first call, default 20 sec. delay-randomize - add random time 0...dr to delay, default 10 sec. retry-time - time between two dialup attempts, default 30 sec. max-time - how long to try alltogether, default 300 sec. modem-init - modem init string. Default ATQ0V1H0 OK AT+FCLASS=0 OK speed - port speed, default 38400 dial-prefix - dial prefix, default ATDT0WP... autobauding - change baud rate to CONNECT "xxx", default FALSE prompt-waittime - pause (in ms) after CONNECT, default 300 nodetatch - don't fork(), default FALSE debug - debugging level, default LOG_LEVEL