You most certainly can use an If...then...Else field to insert the contents of a MergeField

For your example, insert the following mergefields and text

«OKtoPublish» = True «PhoneNumber» ""

then, select the above mergefields and text and press CTRL+F9 which will enclose it in a pair of field delimiters

{ «OKtoPublish» = True «PhoneNumber» "" }

Then insert the word IF as shown below

{ IF «OKtoPublish» = True «PhoneNumber» "" }

Then press ALT+F9 twice to toggle of the display of the field codes in the document.

Note: You MUST use CTRL+F9 to insert the field delimiters. It will NOT work if you insert the directly by the use of the { and } keys.