I watched the episode 9 of the Steins;Gate series and have been wondering how Feyris sent her message in the past from her apartment if the phone microwave was miles away in the laboratory. Until now, they had always sent the D-mails from a phone which was very close to the phone microwave which (if I understood it correctly) managed to open small Kerry blackholes, allowing the electromagnetic signal from the
sending phone to time-travel and reach the
receiving phone in the past.
In this episode, it seems that they can send the D-mails from any location and if the phone microwave is running, the messages go to the past, which doesn't make sense because these Kerry blackholes should extend far beyond the laboratory and then any other (under 36 characters) message sent from a mobile phone in the town (and beyond) would travel to the past and reach the targeted phone back in time (if the number already existed), causing multiple time changes and not just the one caused by Feyris' D-mail.
I really can't understand this plot hole and it ruins my enjoyment of this series.
Could someone explain to me what I didn't get about the functioning of the phone microwave that makes the episode 9 logically believable?