Travel expenses are your ordinary and necessary expenses while traveling temporarily away from home overnight in your ministry.

To me, they are necessary expenses.

Would this cost of mobile calls not be justified as necessary expenses, given the situation?

Of course, we need to hold those authorities to account and make sure that we are paying for only necessary expenses.

Then, it lists three things that are ordinary and necessary expenses, none of which expressly or implicitly would include the work clothes at issue here.