The 45-year-old was last seen next to the River Wyre in St Michael's on Wyre, Lancashire, on a dog walk a week ago.
A major search for Ms Bulley continues, but no trace of her has been found.
Lancashire Police said its "main working hypothesis" was that she fell into the River Wyre and this was "not suspicious but a tragic case of a missing person".
Ms Bulley, a mortgage adviser from Inskip, Lancashire, vanished while walking her dog after dropping off her daughters, aged six and nine, at school on 27 January.
Supt Sally Riley said the last confirmed sighting of Ms Bulley was at 09:10 GMT when she was seen on the upper field.
Officers were alerted to her disappearance when her spaniel, Willow, was found running loose off Garstang Road about 25 minutes after she was last seen by another dog walker.
At 09:20 police believe her phone was on a bench while connected to a work Teams meeting, which ended 10 minutes later.
Detectives believe Ms Bulley vanished in that 10-minute window.
"She puts her phone down to go and deal with the dog momentarily, and Nicola may have fallen in," she said.
"We assume the dog didn't get into the river, but we don't know why Nicola may have if she did."
Ms Riley said the dog was dry and that Ms Bulley could swim.
Search continuesIMAGE SOURCE,PA/PETER BYRNEImage caption,Ms Bulley's phone was found on the bench (top left) where police continue to searchMs Bulley has lived in Lancashire for 25 years but is originally from near Chelmsford, Essex, and has a southern accent.
She was last seen wearing an ankle-length black quilted gilet jacket, a black Engelbert Strauss waist-length coat which was worn underneath the gilet and tight-fitting black jeans.
She was also wearing long green walking socks tucked into her jeans, ankle-length green Next wellies, a necklace and a pale blue Fitbit Supt Riley urged people to "pay heed to those very specific clothing descriptions" and advised the public to "keep themselves safe" in the search for Ms Bulley.