People will believe a bullshit rumour easily if it conforms to one of their preexisting biases.
Anyone who believes that Haitian immigrants are eating the neighbourhood cats is 90% likely to be straight up racist even if they don’t think they are, or 10% likely that they are just dumb as bricks with no critical thought capabilities.
You would have to guarantee those phones would eventually end up in the right person’s hands.
Otherwise if you sabotage 100 phones and only 30 go to your targets, even if you only detonate those 30 there are now 70 phones out there randomly floating around with enough explosives to kill some random person.