By default, the Phone number field on Project record are manually entered. See SuiteAnswers article Creating a Basic Project Record, Answer Id: 9321.
This is the reason that most of the Phone numbers on the Customer record is the company's trunkline. This is to ensure that any phone communication are directed to the correct phone and/or extension numbers. Users may create a custom Entity field that sources and automatically populates the assigned Customer's Phone.
1. Navigate to Customization > Lists, Records, & Fields > Entity Fields > New.
2. Enter a value for the Label field. E.g. Customer Phone
3. Set Type to Phone.
4. Unmark the Store Value checkbox.
5. Under the Applies To tab, mark the Project checkbox.
6. Under the Display tab > set Subtab to Main.
7. Under Sourcing & Filtering, set Parent for the Source List field and Phone for the Source From field.
8. Click Save.