not everyone would be comfortable taking the jack off and reversing the wires and keep in mind if its reversed at their premise and they reverse the wires in the jack someone may correct the reversed polarity on the telco side in which case the phone would stop working again
I used to carry the polarity guards with me explain the situation to the customer and offer to install the guards for a permanent fix
do a google search for polarity guard and you might find a better deal that was just the first one that came up for me
