Christine said that she'll be "happy to pop in later and try to answer any follow-up or new questions", so keep them coming!
I’d classify that as a mistake on behalf of the writer. Just because a writer put something in a story doesn’t mean it has to be “accepted” (especially over almost six decades of many, many scenarios which contradict each other - a lot, sometimes within the same issue). Phones do not pick up heartbeats. Furthermore, the phone has just one microphone which means that Matt cannot remotely tell the difference between left and right. I wrote about this scene here, many years ago: https://www.theothermurdockpapers.com/2010/01/wacky-power-18-psychic-hotline/
I’m also going to take this opportunity to plug my book. It should answer pretty much 98% of all the questions people might have about what does and doesn’t work. Daredevil is a relatively believable superhero, but writers absolutely misunderstand things and make mistakes that may very well be unintentional. https://www.amazon.com/dp/919879650X