Recording the conversation, is not in the functionality. Having said that you can initiate a recording from the phone but that has nothing to do with 3cx. Some phones like the AT-530 allows you to record the conversation by pressing a button but this can only done by the end user of the phone.
So if you after ears-dropping (that is how it is called) you cannot do that with 3CX as far as I am aware off. There are ways arround it.
Appart from that I do not believe it is actually "legal" in some coutries, but than again........
Interesting feature Its possible technically, it work as a forced conference call kind of. We are working on call conferencing for later on in this year.
We will consider this for a future release, especially if more users request it.