Martin Chun, 59, denies causing the deaths of passengers Margaret Luxton, 59, and Carol Muldoon, 68, in May 2014.
He made a series of calls, and was in the middle of an eight-minute call when the coach crashed, prosecutors said.
The calls had "truly horrific" consequences, Truro Crown Court heard.
More on the trial and other stories
Jurors heard the coach had been carrying 51 passengers on an Age Concern trip when it crashed on a steep hill at Morval, near Looe, Cornwall.
Describing the moments before the crash, passenger Maggie Morris told the court the coach "was so fast coming into the bend it was scary".
Mr Chun, from Whitestone near Exeter, denies one count of causing death by dangerous driving and an alternative count of causing death by careless driving.
He has pleaded not guilty to a third charge of causing serious injury by dangerous driving.
The trial, which is expected to last at least a week, continues.
A coach driver accused of killing two people and seriously injuring four others in a crash was using a hands-free mobile at the time, a court has heard.