What do we do???

Look at your divorce decree and if you are liable, you can either pay it or your ex may be able to return to court to get enforcement and require you to pay. If the issue was not addressed, either you can both agree on something or one of you will need to return to court for guidance.

Contact the utility company and tell them to speak with you exclusively in the future. Inform them that you are divorced and no longer have any relationship with your ex-husband. 

Usually, a third-party debt collector can contact only specific people regarding a consumer debt. Under the Fair Debt Collection Practices Act, a collector generally cannot discuss the debt with third parties, except in limited situations, such as contacting someone only to obtain location information, or speaking with certain permitted parties. If the collector gave your ex-husband information about a utility debt that is only in your name, that could be a problem, particularly if your ex-husband is not responsible for the account and was not contacted just to verify location information.