Yes, they can change their phone number. But please note that they can only pay for TurboTax Online using a Canadian or US credit card or pre-paid credit. Or they can download the app on their phone, and use Apple Pay or Google Pay to pay for their return.
To change your phone number, log in to TurboTax Online, open a return and then select "Account Settings" from the bottom of the black left side menu. This will open another page for the Intuit Account Manager. There select "Sign In & Security", then click on "Phone" and you will be able to change your phone number.