I'm working on a web application that will be used internally in an organization. The application will ask users for multiple pieces of contact information. We will allow them to optionally enter a mobile phone number. We also will ask for a phone number to which they would like to receive text messages.
It is likely that the number they would wish to receive text messages on is their mobile phone number. However, in the event that it is not, we would like to ask them to enter a text message number as well.
I could see designing this such that the user enters a mobile phone number, and the user has the ability to indicate (via a checkbox for example) that the text message phone number is the same as the mobile phone number. Doing so could disable the input text box for the Text Message number, and populate it with the mobile phone number that was entered. Unchecking the box could make the input text box editable again and remove the auto-populated value. One flaw I see with this design is that, since mobile phone is not required, checking the "Same as Mobile Phone" checkbox if Mobile Phone is empty would be useless and potentially confusing if the field becomes disabled.
Is there a better way to do this, or another way you have seen it done? Is it easier to just have the user type in the number a second time? Ideally we would like to reduce the amount of manual entry, while also reducing the likeliness of error (user typing in the number incorrectly).