Usually noone called the Doctor. Liz contacted the brigadier to convey her greetings, so the only one not-from-UNIT who might have an idea to call — was the Master.
After he had almost strangled the Doctor with the wire, Lethbridge-Stewart ordered to remove the telephone from the laboratory to play it safe, minding the Doctor's wacky habit to solve everything by himself. But Jo quietly brought it back. Though the Doctor hasn't got talent to hypnosis, he was good at persuading — his smile and natural charm worked smoothly.
"Doctor, the call," the Doctor looked up from the microscope. "It's the Master."