It sounds to me like you have answered your own question.
You don’t like to be on the phone that much – so don’t!
If you are worried about people dropping out of your training
after a month or two, you could require that they do an
ongoing payment each month of X dollars for 6 months or more,
or less, or whatever you think is the right number.
You could also have them fill out a short questionnaire
that might tell you where they are at, and set things
up from there (e.g. 3 month ongoing, 6 month, 9 month, etc.)