I really hate giving my phone number to any service, especially since i might need a second account there, or they might sell my data (or get hacked).
I also hate sites that force you to do so... eg. twitter, where it decides it won't let you register without a phone number (making a new twitter account for a project), then won't allow you to use the phone number, because you already used it for your primary account.
Coincidentally I just made my first Twitter account. I can't remember the exact boxes you need to click but there is a way to sign up without giving out your phone number.
The issue is that it may flag your account as more suspicious. After three days I had my account flagged. I couldn't view tweets or use the account in any way. On log in it would redirect me and ask for my phone number for "verification". Funny how they can verify something they never received. It took about ten days for support to correct but my account is active again and they still don't know my phone number.
I also hate sites that force you to do so... eg. twitter, where it decides it won't let you register without a phone number (making a new twitter account for a project), then won't allow you to use the phone number, because you already used it for your primary account.