Jesus, have you actually talked to the employee at all about their performance yet? I’m not sure what industry you’re in, but there’s a good chance they believe they’re meeting your expectations. If they aren’t, you should start with a sit down to let them know what you would like to see improved. Like the other poster said, it is a good idea to have a paper trail, so maybe make them sign something during this meeting to demonstrate that the meeting was held and expectations were put in place.
With such an immediate firing, I would err on the side of over-documenting than under.
As a followup, I’m sure phone time is hurting their output, but is it possible you’re holding the new hire to the standard your longer employee is held to? Being able to meet the quota of a veteran employee after only 5 days on the job tells me either the quotas are set too low, or your expectations of your new hire may be too high.