This is how you can determine what the lead’s phone number is when an agent is in a call.
There are two places where you can see the phone number of your current call
When on a call, the Lead ID with it's phone number is shown in the upper left hand corner of the agent interface. This is helpful when your Admin has turned on the Script feature that pops up when a call is received which covers the lead info sheet.
You can also click the Lead Tab to open the lead info sheet and you will see the phone number in the Phone field.