Someone want you to enter phone number so they send you SMS for verification purpose.
You can use several free services that have phone number and show you SMS received to that number. Here:

http://www.receive-sms-online.info
https://www.textmagic.com/free-tools/receive-free-sms-online/
https://smsreceivefree.com/
https://www.freeonlinephone.org/
http://receive-sms-online.com/
http://receivesmsonline.com/

Or google: free sms number online