This has apparently been confirmed by Royal Mail. The Trading Standards Office are making people aware of the following scam:
A card is posted through your door from a company called PDS (Parcel Delivery Service) suggesting that they were unable to deliver a parcel and that you need to contact them on 0906 6611911 (a premium rate number). DO NOT call this number, as this is a mail scam originating from Belize. If you call the number and you start to hear a recorded message you will already have been billed £15 for the phone call.
If you do receive a card with these details, then please contact Royal Mail Fraud on 0207 239 6655 or ICSTIS (the premium rate service regulator) at www.icstis.org.uk .