Lines--Each corresponds to a phone number (or extension) that others can use to call you. Your
Cisco IP Communicator can support one to eight lines, depending on configuration. To see how
many lines you have, look at the right side of your phone screen. You have as many lines as you
have phone numbers and phone line icons: 
.
Calls--Each line can support multiple calls. By default, your Cisco IP Communicator supports four connected calls per line, but your system administrator can adjust this number according to your needs. Only one call can be active at any time; other calls are automatically placed on hold.