Getting a phone number isn't fool proof but it does thin out the scammers. The scammers that have contacted me when I've placed on line ads have never given me their phone numbers. They've come up with some interesting excuses for not giving me their numbers. One informed me he was born deaf and couldn't use a phone, another said his wife was in the hospital in intensive care and he couldn't use his phone there but he was able use his computer to email me.
There are so many warnings out there for so many years almost everyone should have heard about them by now. Even if a person isn't compute savvy they should have relatives who are and must have heard the stories. But there still are some out the who haven't learned and won't listen. My wife's uncle, who is in his late '80s, moved back to our home state after his wife died. He set up an apartment and bought his first computer and went on line. Within two years he has given away everything he and his wife have worked over 60 years for and has alienated all his relatives by borrowing money to give to some 40 year old woman he met on line. He's living on assistance from the government now. It's sad but he wouldn't listen to anyone who told him this "woman" was a scam. I doubt he ever met her, "she" was probably some guy sitting at a computer in Nigeria.
Bottom line, if it sounds too good to be true it probably is.