They could modernize the play by making the death the result of a typo in the text message. E.g. Juliet cried / died
"Oh, I am autocorrect's fool!"
Did you see the Baz Luhrmann take on R&J from the 90s? The missed message was represented by a FedEx "Sorry we missed you" note.
Only if their parents hadn't taken their phones away from either of them, and the SMS went through immediately without fail. It's rare, but the latter still happens to me sometimes even when the network appears to be working fine otherwise.