 A small number of modems added the ability to automatically answer incoming calls, or automatically place an outgoing call to a single number, but even these limited features were relatively rare or limited to special models in a lineup.When more flexible solutions were needed, third party "dialers" were used to automate calling, normally using a separate serial port to communicate with the dialer, which would then control the modem through a private electrical connection.The introduction of microcomputer systems with internal expansion slots made the first software-controllable modems common.