"just because it can appear that way" is all "cause and effect" are.
The source doesn't do the math on the final step, but you can arrange the third observer so they emit light that reaches Earth before the phone call is sent (because we are assuming FTL tech).
The source doesn't do the math on the final step, but you can arrange the third observer so they emit light that reaches Earth before the phone call is sent (because we are assuming FTL tech).