On your iPhone 12 or newer, running iOS 17, go to Settings then Accessibility.
The 4th header is SPEECH. Under the SPEECH header, there is Personal Voice. Select "Personal Voice".
On the Personal Voice screen, there is a button for "Create a Personal Voice". When you select that, you will be given instructions.
You give the personal voice a name (the default be something like "Paulo's Personal Voice 1") then it does a check to make sure the conditions are okay. If it's too loud in the current environment, it will probably mess up the recording.
When I turned on Voice Over to test this just now on iOS 17 beta 6, the Voice Over instructions were read from my phone's ear speaker so it would work like a phone call: Voice Over reads the instructions and you response like a phone call but my face hit the screen so that messed up the Voice Over selection. I think it would require AirPods or other headset with a microphone so you could control voiceover while hearing instructions and speaking as needed. I don't have any handy so I can't test it with headphones.
You have to read 150 phrases. You can stop at any time and continue later. After you repeat a phrase back, it will automatically move to the next phrase after a second or so.
I hope this helps. I can test further if you have any questions.