“Hi Amy. This is Zack.”
You're having a meeting with your coworker Brenda in her office. You think of a question that you want to ask another coworker, Amy, about, so you call Amy from Brenda's telephone. When Amy answers, you say this.
Hi Amy. This is Zack.
On a telephone call, identify yourself with "This is ___":
Hi, this is Aaron Knight. Can I speak with Mr. Jones please?
Don't say "I am ___." That's a good way to introduce yourself when you meet someone for the first time in person, but it's not correct for telephone conversations.
When someone asks to speak to you on a telephone call, and you want to tell them that it's you, use "This is he/she.":
A: Hi, can I speak with Masahiro Tsukada?
B: Yes, this is he.