NO DEBT INFORMATION ?

Received a demand, claiming to represent ER Consiulting, from a company, called NUDEBT - is this a ****? NUDE DEBT ?? They claim an amount is owned to ER Consulting but have not provided any invoice or details of any service provided. They allege this is for my Sister, a full time resident at a Care Facility and for whom I am a court appointed Administrator. I googled ER Consulting who appears to be an Ambulance Service? their (unhelpful) call centre representative refused to put me through to a senior person to obtain more information about this alleged "debt" on behalf of my Sister.

What do I do now.