"in a phase of" is correct and usable in written English.
You can use it to refer to a certain state or period in a process or development. For example, "We are 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.
(2007), these reforms ushered in a phase of more stable growth of the economy.
"I'm in a phase of heavy research," he said.
He's arrested in a phase of life, the social phase".
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.
Graphic design is in a phase of deep conservatism, fighting a rear-guard action against the digital onslaught.
The ASO confirmed that they are "in a phase of reflecting on the matter raised by Emma Pooley [and others]".
"That's right," she said, and added, "It's true today that we're in a phase of accession to power".
We are now in a phase of retreat from easily readable content.
We're in a phase of very open-minded, versatile castings," Ms. Joos says.
