It's common to say, "Hi, this is Bob and Sally."
There is nothing grammatically wrong with saying, "We are Bob and Sally," but Americans do not normally identify themselves that way on the phone. When it's just one person, you don't say, "Hi, I am Bob", you say, "Hi, this is Bob".
Note that if you are introducing yourself in person, you do say "I". That is, if you call someone on the phone, you will say, "Hi, this is Fred Smith," or "Hello, this is Bob and Sally." But if you go to his office to meet him, you say, "Hi, I am Fred Smith," or "Hi, we are Bob and Sally." There is no grammatical reason for the difference: it's just convention.
I suppose one might say that it should be "these are" because it's plural, but "this is" is commonly used in English to identify a collection of things that are functioning as a unit. "This is your desk and chair", "This is our plan and goal", etc. But if it's more than one of the same thing, you do say "these". Like if you were introducing a new employee, you might say "These are your co-workers." But if you name them individually, you typically say "This is Fred and Mary."
If you are giving additional information to identify yourself on the phone, you typically shift to saying "I". For example, "Hello, this is Fred Smith. I have an appointment on Monday."
"Classical Mechanics is a well known field of Mathematics." Do you even listen to yourself...? – Najib Idrissi 19 hours ago
The user "Najib Idrissi" asks the author of the sentence Classical Mechanics is a well known field of Mathematics. to read that statement for themselves. Najib makes sure that the author will realize the scientific mistake in that sentence and, in short, "will come to their senses."
It's like your friend has told you something very oddly wrong, and you didn't expect him to say so. You tell him/her to listen to himself/herself, so that s/he will realize the mistake they've made. In this case, it seems very rudimentary to the mathematicians that "classical mechanics" are a physics' subject, and hardly related to math. So, "well known", as it means that "many are familiar with it being a part of mathematics" seems a very idiotic expression to them, as if the author didn't know what they were typing, or simply, talking about.
If you ask me, there is a bit of negative connotation in this sentence, but it wouldn't be considered as offensive. (As "offensive" is too strong for it)
In fact, the expression "listen to yourself" is commonly used in the area of philosophy when there's a speak of art of communicating between humans.
philosophy reference 1
philosophy reference 2
The third
And this is a nice example of a similar usage. (Though it contains the expression and not the exact question)
in Google books
Best Answer
This is X (or just It's X) are more natural when introducing yourself to someone who knows you, but doesn't know you're the one speaking or texting.
I am X or My name is X are - in my opinion - better used when introducing yourself to someone who doesn't know you, or doesn't know your name.
A good way to remember that is to consider which question you've been asked - an English speaker would typically ask *Who's that?*or Who's speaking? in the former case, and Who are you? in the latter.