Double check validations on learners tested

https://praekelt.atlassian.net/browse/RZ-20?jql=project%20%3D%20RZ%20AND%20resolution%20%3D%20Unresolved%20AND%20priority%20%3D%20Critical%20ORDER%20BY%20key%20DESC
Change all to `self.check_valid_number(content)`