hi
You won't be able to access the account phone number directly via user object
Basically the {{ user }} object will have all attributes for the Contact entity
parentcustomerid is an attribute of type Entity Reference, so you can access the Id and Name using dot notation
to access more of Account data, you will have to to a simple retrieve to the Account entity, as you already have the id
try the following, or something link the below.. and then you will have access to the Account attributes
{% assign account = entities.account[user.parentcustomerid.Id] %}
Take a look at the documentation for a more information:
https://docs.microsoft.com/en-us/powerapps/maker/portals/liquid/liquid-objects
