I am trying to replace blank text field values using the following expression.
For some reason this formula does not work and blank rows were not replaces with 'Texting'.
It may be that these are not null values on a row (where the if(isnull syntax would work) rather places where there are no values due to associations between tables not joining up?
In these cases you can't easily populate values, but there are usually ways around it.
Can you please give a bit more detail about your data model and where you want the Nulls replaced?