Press up twice. From there, dial these number to spawn a vehicle.

Jetmax - 938-555-0100
NRG-900 - 625-555-0100
Sanchez - 625-555-0150.
FIB Buffalo - 227-555-0100.
Comet - 227-555-0175.
Turismo - 227-555-0147
Cognoscenti - 227-555-0142
Super GT - 227-555-0168
Annihiliator - 359-555-0100
Helicopter - 359-555-0100
Police Charger - 227-555-0100