Hi,
Thanks for using our plugins.
I can try to help out here, but I am only able to support existing features and may not be able to provide much for a customization.
There is a good article someone wrote on this that might get you most of the way there:
https://nielsvr.com/2015/12/07/extending-the-events-calendar-attendees-csv/
George has the basis for a system here too:
https://theeventscalendar.com/support/forums/topic/add-attendee-telephone-number-to-attendee-list/#post-953675
If you think that is something you would like as a feature I encourage you to visit our User Voice Page and either upvote an existing request or make a new request there.
http://tribe.uservoice.com/
Let me know if you have any follow up questions.
Thanks