If the customer interrupted you, you had an obligation to continue the min-Miranda but understand you could do that by simply continuing to speak and that may even mean speaking over the customer. Further, you could call back later or on another day as long as you are in the confines of the Fair Debt Collections Practices Act. You could also leave the message on the answering machine but I believe the issue there would be you may need to include certain disclaimers to avoid privacy law violations.