https://telegram.org/faq#q-what-39s-the-difference-between-g...
It took me more than 10 minutes to figure out how to add a friend with a phone number from Spain. The trick was adding a space in the right place. In the meantime, she couldn't find me by my US number.
Turns out, different countries format the phone numbers differently, and Whatsapp gets confused. Just for US numbers, you have +1(xxx)xxx-xxxx, xxx-xxx-xxxx, 1xxxxxxxxxx, xxxxxxxxxx,(xxx)xxx xxxx, xxx xxx xxxx - and I don't know which one Whatsapp uses internally for ID.
For what it's worth, entering the phone number exactly as it shows up on my account info didn't work.
TL;DR: a phone number does not a good ID make.
Then there are places like China which no one uses WhatsApp.