In the US, "PanPan" may just get you puzzled looks/inquiries, because it is seldom used. If you need assistance, call "Mayday".
In the rejected takeoff scenario, it depends on whether it is a simple failure or fire/severe damage. If you can clear the runway and taxi back to your gate, simply state your intentions. If you were near V1, which means you will have hot brakes and/or flat tires, you should call Mayday to get the fire response moving. Same if you think you spewed your turbine blades all over the runway; an inspection/cleanup before the next attempted takeoff is warranted.