An update had been sent by Google to all the advertisers on March 31, 2015 which was related to call extension in Google Adwords. According to this update the phone number which are being shown in call extension will have to be verified and this starting from June 2nd, 2015. If phone numbers which will be unverified, call extension will be disapproved.
Have a look on official Email :
As mentioned above in the email, the phone number can be verified:
1) Having on your site landing page
2) Link between Google Webmaster and Analytics
3) By adding Adwords conversion code to ad's landing page.
As you read, the first two options are seems to be so easy for verification and last one will be the least popular option.
