Get calls from this number but when I answer they hang up

Caller: Careflight

This is Careflight, they are an honest and reputable organization, they provide emergency medical assistance by air flight. They run using donations but rarely ring random people for donations, usually only ring people who have already expressed an interest in donating money.

I have received 6 calls this year. No message is ever left.

they call almost everyday and do not leave a message, i tried calling back & it is busy

I called back number (02) 8020 3509 rang 2 times then goes to busy tone.