If most of your calls are fine, but you’re experiencing in-call connection issues with just a few students (perhaps even the same ones over and over) it’s safe to assume the issues are on the students' sides.
In situations like this, the best thing to do is to direct your students to Student Support ("contact us" on the students' side). They're excellent and can resolve all sorts of matters nearly always in the students' native languages.
We hope that helps you to have no more in-call issues!