The offices were closed on Sunday so who were Tim and Tom ringing on Sunday?
I can't quite explain why but we generally don't say "where" when we're talking about a phone call. Generally, you're not phoning a place, you're phoning a person, even if you don't know who's going to answer the phone.
You got the word order right in your title - Where were they making calls to?
You got the word order wrong in your post - Where Tom and Tim were making calls to?
That's a careless error. Clearly you know the right word order.
Student or Learner