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."