Not sure about 18 digit numbers. But it's more than likely spam. Considering most of such calls are being made from PCs it doesn't seem out of the realm of possibility for them to have more numbers in their caller ID. You could do what I did and download YouMail (as my prepaid carrier doesn't offer visual voicemail). Its a free app and it catches a lot of spam calls for me and even puts any of their voicemail into a spam folder.