Do you hear the noise on the speaker phone or through the handset? If the later try replacing just the handset and cable, or try using a headset instead. Still the same?

If the sound is coming through the speaker phone, then it is likely the phone itself is faulty and RMA will be necessary.

If you connect the phone to another network point and the problem moves with the phone, then at least you know the problem is definitely the phone.

Hope this helps.

Allan.