How to authenticate user using mobile number in Keycloak

Extending the server
,
#1

User has a custom attribute phoneNumber in Keycloak.

There is a default method for fetching token using username and password but would it be possible to authenticate using phoneNumber / password instead of username / password

curl \
  -d "client_id=$CLIENT_ID" -d "client_secret=$CLIENT_SECRET" \
  -d "username=$UNAME" -d "password=$PASSWORD" \
  -d "grant_type=password" \
  "$KEYCLOAKHOST/auth/realms/$REALM/protocol/openid-connect/token"

What call should I use to authenticate using a custom attribute in UserModel