I have done this for many years and it's failry straigt-forward.
You still use the default Username field, but change the text to "Cellphone".
There are probably an easier way to do this, but I haven't had to research it since this has worked since the Amigopod days :).. The default password field is always pre-filled in the default receipt form. What I did was copy the html elements needed from the receipt page, then checked the "Do not include the Guest receipt contents". Then I entered the html <form elements needed which is basically username with value of
{$username}
, password type tet-field, mac -adress and some more fields needed and the submit button.
That way I got the username pre-filled, and the user only had to enter the password to login..
Or - you can remove the password field and login button from the Receipt form, and add a link to the login page.. You can send username as a field to that loginpage in the url like
.php?username={$username}
to prevent them from having to input that again.. That adds one extra page to navigate, but thats a but easier than doing html if you're not comfortable with that.