Impatient.
They are called Umpires. One is the plate umpire, one is the field umpire.
Its depends like if he is dialing in call center then he is a working as a "Caller" or "Telecaller" or "Agent". Managing callers under him then "Team Leader" or "Team Manager"
A person receiving a telephone call is a "callee". http://www.merriam-webster.com/dictionary/callee
tell them to stop calling at school or just call them after they call you :]