The trapped farmer then used the phone to call a workmate who was able to lift the tractor from his injured leg.
It was thought the farmer's leg had been trapped for 15-20 minutes and, if he hadn't reached the cellphone, the result may have been different.
He suffered crush injuries and major fractures to one leg, foot and ankle.
He was treated at the scene with pain relief and transported to the Bay of Islands Hospital in Kawakawa, where he was stabilised and X-rayed before being transferred to Whangarei Hospital.
The man is expected to undergo surgery to his lower leg, ankle and foot.
"If his cellphone hadn't been so close, he wouldn't have been noticed as missing for several hours and would have lost his leg," the St John spokesman said.
"He's a very lucky chap. If the phone had landed a few centimetres further away, it would have been a different story."
A spokeswoman for the Ministry of Business, Innovation and Employment said it had been notified of the accident and had launched an investigation.
Its health and safety inspectors had visited the scene, she said.
MBIE figures show that from 2003-2008, more than 70 per cent of agricultural fatalities involved a farm vehicle - of which tractors and quad bikes accounted for the majority.
Most accidents and injuries are caused by roll-over events due to uneven or dangerous terrain, unbalanced load or towing, rider inattention.
