"in a phase" is a correct and usable phrase in written English.
You can use it to refer to a particular stage, period, or point in a process. For example, "We are currently in a phase of rapid growth."
L'Avventura ushered in a phase of Antonioni's work for which he is well known and well parodied.
"I'm in a phase of heavy research," he said.
He's arrested in a phase of life, the social phase".
"We are in a phase heading nowhere," he said.
Ferguson's United are in a phase when even the flattest of landscapes contains terrible hazards.
This is already in a phase scientists call "commitment" – meaning "unstoppable inevitability", McCalman writes.
I was in college, in a phase of obsession with postmodernist fiction.
The state "is in a phase of deliquescence," said a former justice minister, Carlos Vamain.
"Now we're in a phase where there may be a decent outcome," he said.
"We judged that we are now in a phase that requires additional easing measures".
"We are right now in a phase where everybody's preparing contingency plans," Mr. Beerli said.
