I have ever used a caller, just a personal thing to memorize the test, but, as with what others have said, using a caller is perfectly legal at First Level but a few things...as a scribe I have heard several readers and they are not all the same. Some read way too far ahead..they read a good two or three movements ahead of where the rider is currently in the test and at least for me, my concentration would never remember what was read that far ahead. I saw several riders go off test even with a reader specifically for that reason. At the same time, other readers are so slow they are essentially "with" the rider step by step to the next movement..it doesn't give the rider a chance to prepare.
As the occasional reader, I watch the test and read off the next movement roughly halfway through the current movement, and if it is a block with a lot of verbiage I may break it up into two separate movements in the read. As said, it is in the timing and what you feel (think of how nervous you think you are going to get) you can remember depending on how far ahead the reader goes.
My recommendation is to memorize your test, practice it; it actually doesn't take all that long..practice it in sections if needed..you don't need to memorize the entire test at once the first time you ride it. I have blown up versions of my tests that I tape to the arena wall at eye level (mounted eye level) so I can refer back to it if need be. While warming up your horse at the walk, walk the entire test and I do this whenever learning a new test; you can verbalize the gait..trot here, canter here etc.