We get that over here, the message usually says however, please say if you do not wish the discussion to be recorded. But yes, if you do not specify, you simply do not know if they recorded it or not.

In the early days, because both microphones and speakers are magnetic based, if one is too close to the other it can cause a hum or interference. These days of course there's many more technologies. The phone line could pass through a recording device before it reaches the actual handset (if a handset is used.) Digital signals make a difference.