By default the Linksys SPA range of handsets have call waiting enabled, this means if you are on a call and a new call comes in you will have a chance to put the current call on hold (or terminate it) and pick up the second incoming call. To disable/enable this feature use the feature as below.

Type in the star code below and hit dial:

*56 Enable call waiting on all calls
*57 Disable call waiting on all calls