If you won't ever be using discount codes, you could create a dummy Discount Code (Admin > Cart Discount Settings) to enable display of the Discount Code field in your cart, but then change the "Discount Code" text label in the cart screen by adding these lines to your View Cart button code, just before the "// -->" line:
function EJEJC_config() {
EJEJC_POSTCALL=true;
}
function EJEJC_shown() {
jQuery("#dscnt_cd").attr("innerHTML", "Your Phone Number");
}
NOTE: If EJEJC_config () {} already exists in your code due to other customizations, then simply add the relevant lines to it.