Try entering any of the following codes after Roman has given you his old mobile phone.

Full Health:
Call 362-555-0100

Full Health and Ammunition:
Call 482-555-0100

Reset Wanted Level:
Call 267-555-0100

Wanted Level Up One:
Call 267-555-0150

Weapons 1:
Call 486-555-0100

Weapons 2:
Call 486-555-0150

Change Weather and Brightness:
Call 468-555-0100

Spawn Annihilator Cop Helicopter:
Call 359-555-0100

Spawn Cognoscenti:
Call 227-555-0142

Spawn Comet:
Call 227-555-0175

Spawn FBI Buffalo:
Call 227-555-0100

Spawn Jetmax:
Call 938-555-0100

Spawn NRG-900:
Call 625-555-0100

Spawn Sanchez:
Call 625-555-0150

Spawn SuperGT:
Call 227-555-0168

Spawn Turismo:
Call 227-555-0147