Yes, you'll be able to receive SMS to open a WhatsApp account on the country-code of your choice.
It can be due to, maybe many people already registered and using this number at same time, or there cross the limit of uses this number.
No, It is completely safe and secure to receive and SMS. but keep in mind don't share your OTP or SMS with others.
Yes, you just have to pick a number you want to verify, then you will receive OTP on the website, now you can easily set up a Gmail Account using temp number.
You just have you fill the number in you WhatsApp app and send OTP, then you will receive an OTP SMS on this website. Fill that OTP for complete verification.
There are so many reliable websites to get temp number after paid, but if you want free numbers then visit temp-whats-numb.online.