XF 2.1 BB Code to return value of custom user field


I want to create a BB Code which returns the value of a certain simple custom user field.
I have a staff list as a thread on my internal Forum and I want to include the telephone number of the staff members. (Field ID "user_tel").
I figured I need to use the PHP Callback function on the BBcode, handing over the requested UserID as a parameter.
Could look like this: [GETTEL=1][/GETTEL]

So far I have figured that I get my wanted result with $user.Profile.custom_fields.user_tel but then I got stuck on how to get the $user by ID.
To be quiet honest, I don't know how to proceed either so I'll gladly take the help as detailed as possible 😅

Thanks in advance!
I'm having a hard time trying to wrap my head around the XenForo syntax/Callback methods/Where to start in general.
After reading the /xf2-docs/manual/, Developer Doc and several other threads/resources I came to the conclusion that I need help on where to get started as they do list everything, but lack the sort of explanation I need.

As a side note: I couldn't find a beginner friendly doc/tutorial/etc. on getting started. I'll happily take all the helpful links some of you might have bookmarked.