@Gabender
Problem is that
Phone number field when posting new lisiting is item related, but user's one is user related.
Unfortunately you cannot catch 1 custom field (or I do not know about it), but if you would replace i.e. city area field and use it as phone number, you could fill it with user phone.
Alternatively you can use php and javascript. Something like this:
- if user is not logged, do nothing
- if user is logged, run javascript that: hide custom field, add value to custom field
If you can check if this field has unique ID or class, it would be easy to do