On international, the charge is usually 10% of the fare at the time of ticketing for the cabin of travel plus 100% of taxes/fees. Sometimes the price goes up the longer you wait and it can be difficult to get this done quickly at an airport.

If you can do something else around a speakerphone, why not try to wait on hold for an agent, perhaps in addition to trying the callback number option. Sometimes the phone queues go faster than expected.