On reservations lines, regardless of airline, there is a fairly relaxed environment compared with the airport. It's much easier for the agent to limit the options they will offer. At the airport, with the customer standing in front of them the GA has more pressure to find a solution that is acceptable - the customer isn't going to go away.
That's why I'd always advise being in front of an agent, with your bag in your hand, and tell them what you want. You don't tend to run into as many obstacles then. This is true on the new UA, PMUA, PMCO, and virtually all other airlines.