Salesforce CTI save log after call

In the Salesforce CTI API, how does one get the WhoID parameter when saving a log back to Salesforce after a call has completed?

This is the example code:

sforce.interaction.saveLog('Task',‘WhoID=0039000000HVnvh&CallDisposition=Internal&CallObject=TestCall&Description=Test task comments&Subject=Test Call&Priority=Normal&Status=NotStarted', saveLogCallback);

But where does the WhoID come from? As only the phone number gets passed to the CTI application during click-to-call?