A:AnswerYes, You can record audio from an external audio source through the AUX IN JACK . Note : The MCR-100 is a mono player/recorder,
so only the left channel will be recorded through the AUX IN JACK.
A:AnswerYes. You can buy a mono adaptor to plug your headphones into and then plug it into the headphone input of your cassette recorder and you will get mono out of both sides of the heaphones.
A:AnswerRogerif I want to record a message from my iPhone, I turn on the speaker. So of course it would work. I don't use a landline. So I don't know, if you can use it on the landline. I suggest you talk to customer service on that.