What is "callup"

Usage examples of "callup".

I have always counted on my fingers and still do and I had been so nervous about this that I went to classes with the ATC in Darrowby before my callup, dredging from my schooldays horrific calculations about trains passing each other at different speeds and 71 72ALL THINGS WISE AND WONDERFUL water running in and out of bath tubs.

People working at two and three jobs to make ends meet, lives broken by reserve callups, tax evasion as a way of life, sons and daughters leaving the country and not coming back?