hi,
i have an android phone with the alexa app installed. your reminder will only go off two ways. one it will go off on whichever echo device you spoke your reminder to. next, i have an android phone with the alexa app installed on it. when the reminder goes off regardless where i am, my phone gets the notification for that reminder.
as for setting a reminder on your phone to basically do the reverse? a person could take with a fire tablet, that'd work. i have not done that. i just sync the android google calendar and alexa apps. i suppose you could set your echo device down next to a landline answering machine. call yourself with the speaker set LOUD. then phone home, use your voice over the machine to trigger alexa IF we lived in 'macgyver world.'
joking aside, hope you get it all worked out.