Currently it's only possible to sign into the mobile application by using an email address that matches the email address on your employee record.
However, we have launched an early access programme for organizations to test signing in using phone numbers. The programme is best suited for organizations that collect employee phone numbers, and is ideal for deskless workers that do not have a work email address. It's also ideal for customers that use our kiosk survey experience, and would like to provide their workers with access to personal dashboards.
See more details here: https://community.workday.com/node/1121750 (requires Workday Community access). If you're interested in taking part in the programme, you can sign up through the community link or contact your customer success or functional consultant for more information. It's not possible for employees to take part in this programme if their organization hasn't signed up.
The Log in with phone number option displays for all users, because it was not possible to limit the update to only participating customers. This is because the mobile application can't verify whether an employee is in an organization that's part of the programme before they authenticate themselves. However, an error will display for employees of customers that are not part of the early adopter programme who try to login with their phone number.