Called referencing a telephone survey regarding asthma and allergies that I participated in last month. Man on the phone said that I was randomly selected to win a travel voucher for free travel or a "slap-chop"; my choice. To claim my prize all I had to do was allow a salesperson to come to my house and give me a demonstration for an air purifier; the salesperson would bring the voucher with them. I told them I wasn't interested and I didn't want anyone coming to my home to try to sell me something. Person on the phone was very smooth and used various pressure techniques to attempt to convince me. I did a little research online and it sounds like similar scams where they get in your house and try to pressure you into buying their product or get your friends' information in order to get your "prize". Avoid.
Also, the original 'survey' call was from a blocked/unknown number.