I know so many people with two phones. People from the community get the kosher number, everyone else gets the regular phone. WhatsApp is the new modern version of נייס. Let's not kid ourselves here
Hot take: it’s a bad sign if this is the norm. Instead of optimizing the definition of Kosher phone, you have people with a crippled kosher dumbphone and also what is likely an unfiltered treif smartphone 
Not the worst take.
(I happen to be involved somewhat tangentialy in the sugya)
It's takeh a problem.
But depends on each community...
Some are totaly pure only kosher phone, with very little of the second phone in the other pocket, and if yes, filtered well, while others have major issues...
On the other hand to
@LongTimeLurker's point - A bachur who has whatsapp, texting... especially in a way that the army can tie the number to him...Is most likely not a chareidi full time learner "toraso umnaso".