Great questions and ones that I've asked umpteen times myself. Having a call is a common phenomenon among missionaries I know, but not at all universal.

For us, one Friday we received a letter in the mail from an Ethiopian who PS'd that if God ever called us to Ethiopia, he was sure we would have a productive ministry. We threw it in the circular file. The next Sunday, Gary went to church while I stayed home with my elderly and ailing mother. At exactly the same moment, while he was in church taking communion, and while I was at home listening to BBC news, we were both overwhelmed with a burden for Ethiopia. We got together on it over lunch and then called the guy who had sent the letter. He told us that when he met Gary (a couple months earlier), he heard a voice saying that we were being called to Ethiopia.

I think the stories are as varied as the individuals. I'd like to hear some of the others who have been called.

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Mary Vanderkooi
Kale Heywott Church (KHC)
Soddo, Ethiopia