Is it permissible to record a debtor's call without his knowledge to prove the debt, given that there is no other way to prove the right?

You erred by not documenting the debt, and Allah Almighty says: "O you who have believed, when you contract a debt for a specified term, write it down." It is not permissible to record others' conversations without their permission, but it is permissible if there is an overriding benefit, such as proving a right and returning it to its rightful owner.