By the standard of the course this one is pretty straightforward. He could be hearing other peoples cell phones, calls coming in to abandoned pay phones or perhaps he just has auditory hallucinations. ;)
Hearing ringing would make more sense. Or hearing a phone. Better yet, "the phone." But "telephones"? Awkward and very un-Hungarian-sounding. Absolutely zero hits for "telefonokat hall" in Google, unless you count this sentence from Duolingo. Pretty terrible-sounding in English too. And why not "from outside"?