Learn English – How to say email “mis-sent” in a business manner

If your job is such that you would normally be the first person reached when someone calls from outside the company, then you would normally answer by saying something along the lines of your second example (at least giving the name of the company).

If you are not normally the first line of contact, then usually you would answer the phone by giving your particular department (e.g. "human resources") and possibly giving your name, depending on how verbose you want to be.

If you are not normally even the first line of contact within your department, then, depending on your work environment, you could either answer with the department, or just by saying "This is Alex", or "Alex speaking" and possibly adding, "how can I help you?"

Dear is a perfectly appropriate letter greeting in all circumstances. However you do need a noun to follow. "Dear, " on its own doesn't work.

The standard opening if you don't know enough about the reader is "Dear Sir/Madam,"

Note that when used to open a letter, dear is an adjective:

Dear Mr Smith,

Dear Susan,

Dear Sir/Madam,

Dear is also used as a noun, typically in conversational speech:

Yes, dear.

Calm down, dear.

This usage is usually reserved for loved ones -- but this is a completely different usage from the letter opening greeting.