So when I try to activate an iPhone 5S (bought at full retail at apple store, never used/activated before) on the newly added UDP line, it asks me to use my "existing" sim. Is there a way to do it without opening the iPhone 6 that came with the UDP line?
So you want to use the 5s on the line that was created on the 6??? If so it would be best to use the sim that came with the 6