I always abruptly answer in a deep southern accent and bellow:

“In the name of JAY-SUS! The LORD came to me in a dream and told me I was HIS tool to bring one of HIS stray sheep back to HIS flock! Together we we pray, friend, and exorcise the demons possessing your cell phone! Through JAY-SUS we will banish them and cure you if your drug and pornography addiction! Will you PRAY with me, brother?”

Then I get all quiet and start mumbling in circular talk that leads nowhere until they hang up.

Usually solves the problem.