Hi,
you will need to edit file: ~\CMSModules\Ecommerce\Controls\ShoppingCart\MapScriptControl.ascx.cs
In there you will need to change code. If you want to make txtBillingPhone as mandatory edit line 936:
// check billing part of the form
string result = val.NotEmpty(txtBillingName.Text.Trim(), GetString("ShoppingCartOrderAddresses.BillingNameErr"))
.NotEmpty(txtBillingAddr1.Text.Trim(), GetString("ShoppingCartOrderAddresses.BillingAddressLineErr"))
.NotEmpty(txtBillingCity.Text.Trim(), GetString("ShoppingCartOrderAddresses.BillingCityErr"))
.NotEmpty(txtBillingZip.Text.Trim(), GetString("ShoppingCartOrderAddresses.BillingZIPErr")).Result;
your code would look like this:
// check billing part of the form
string result = val.NotEmpty(txtBillingName.Text.Trim(), GetString("ShoppingCartOrderAddresses.BillingNameErr"))
.NotEmpty(txtBillingAddr1.Text.Trim(), GetString("ShoppingCartOrderAddresses.BillingAddressLineErr"))
.NotEmpty(txtBillingCity.Text.Trim(), GetString("ShoppingCartOrderAddresses.BillingCityErr"))
.NotEmpty(txtBillingZip.Text.Trim(),
.NotEmpty(txtBillingPhone.Text.Trim(),
GetString("ShoppingCartOrderAddresses.BillingZIPErr")).Result;
also I think you will need to edit line 323 to mark required field.
Hope this helps... maybe I have missed something....