Outgoing connections are handled by a ZyXEL 1496E modem. This line is generally used to connect to customers.
The diald package is used to manage the part-time links. Diald does the expected wait for packets and bring up the link. Diald is very configurable in the packets that will cause it to come up and stay up, and how long they'll hold the link up. It allows you to run custom commands when the link is setup and torn down. It will also correctly handle dialing into a system that assigns dynamic IP addresses.