I have perused various FT airline forums (e.g., CO, AA, DL, NW, etc.) to see if this problem has occurred with other codeshare relationships. To the best of my knowledge, NO OTHER PARTNERSHIP (save CO/VS) has a problem with assigning seats if the passenger contacts the OPERATING carrier, especially once the reservation is ticketed.

That said, I'm thinking that maybe the AC agents are misunderstanding the nature of the request, especially if the passenger is quoting his or her UNITED flight number or PNR. If the AC reps. are given the AC flight number or record locator, they should have no problem in assigning a seat; if they are given the UA info., they will tell the customer to call United.

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