It's called 'caller id spoofing'. The people calling you can make any phone number show up on your caller id. They're making a number show up that charges you money if you call it and press a certain key.
At the same time they've disguised the messages on their pay by phone system as surveys. Those messages normally tell you what you're buying and how much it costs.
Check your bill for unwanted charges if you called this number back.
Ok, if you received a call and didn't answer and didn't receive a voicemail, don't bother posting about it. People look on here to find out who is calling, not how many of you don't answer your phones and don't know who it is. You're not helping anyone, just saying.