A phone number could be which will be registered to that digital wallet and every time you will do transfer the process should ask a code that is being sent from your phone number in order to proceed.
Using a phone number and SMS as your 2FA is inherently insecure as thieves can quite easily get your phone number transferred to another SIN, rendering the entire 2FA meaningless. You are much safer using something like Google Authenticator.
I dont know whether its true that its very easy to steal code from phone number or not since u just know that is very difficult for me.
Anyway, Google Authenticator is very good so actually by using that 2FA, you canbe sure that your wallet is safe but this is normally in exchange wallet.