I was on the UA site booking a flight that happened to include a partner airline. I wasn't trying to book the partner airline, nor was I on the partner site trying to book. The agent via text told me that I could not use miles to book a flight with United that included a partner, I could only book a United Flight-as in my miles were worthless. She then gave me 2 UA only options, one of which was about 500,000 miles more than I had and the other was to simply pay cash instead of using miles at all.