What is the final step in the Friar's plan? Describe some problems that could arise with this step?

Friar Laurence will write to Romeo and explain his plan. The problem is that nobody had cell phones back then so the monk, who could not get past the town because of the plague, had to turn back. The problem is so many things can go wrong trying to message Romeo. Friar really didn't have a plan B if things did not work nor could he know if things went wrong.