I have found this product to be an efficient and inexpensive aid to recording phone conversations provided I used a corded telephone. It did not work at all on the cordless telephone set I normally use.
So, when making phone conversations I wish to record, I now ensure I use my corded telephone, attatch the sucker containing the microphone to the back of the telephone handset ear-piece, switch on the recorder and make the call. It does the job perfectly well, with both sides of the conversation being recorded with equal clarity.