You are calling an employer. You would say:
I am calling an employer about a new job.
When you are called and asked your example question, you would say:
Your peer: Who is it?
You: It's an employer who has a new job posting for which I applied.
..or any number of other responses to "Who is it?":
It's Jane from accounting.
It's someone from a company to which I applied.
It's a recruiter with information on a new job.
It's my friend Sally.
I'm not sure; I think it's a wrong number.
(or if you'd prefer to use conversational English to talk to your peer)
It's a company that I applied to.
It's someone calling about my job application.
Just a company that wants to give me money.
Oh, I applied to a job posting a while back at this company and they're calling me back now.