Hi, thank you for contacting JustAnswer.com. My name is Russell. I will do my best to provide the right answer to your question.
That is probably a case of a phone that is defective from the vendor or store, unfortunately. A constant 'busy' noise, as you put it, cannot be much else than a defect in electronics, I would think.
But I must ask to be sure of this: When the phone rings from an incoming call, from receiving a call in other words, can you pick up and respond to the call with the handset? do you hear anything but busy signal from the handset at that point?
If even the base unit rings in response to incoming calls, then it is connected to a functioning phone line for sure, I think. But the handset seems to me to be where the problem is.
If the phone is new enough, by all means pack it up again and return it for an exchange as defective. Get another, working phone, for free that way.
(If it is past the return-exchange period, of course, you might be able to get it repaired under warranty terms, for free, if it was bought within a year ago, and depending upon what country it was bought in or from.)
Let me know if I can advise you further on this matter, toward an ideal solution.