I have a merge document (Word 2013) that I want to distribute to customers and have them fill in the missing information. The merge document populates fields from our database but where we have missing entries, in the mobile phone number for instance, I want a formtext field to display, prompting action from the customer to complete.

I have tried using an IF statement:
{ IF { MERGEFIELD "Mobile Phone" } = "" { FORMTEXT } { MERGEFIELD "Mobile Phone"} }

As you might expect, if "Mobile Phone" has data, then it is displayed. But if the field is empty, then nothing is displayed. Other formtext fields not embedded within merge fields display correctly.

How can I display the formtext field conditionally?