I know nothing about this agency so take what I am about to say with a grain of salt....
Sounds like they don't want to schedule something but want a heads up to make sure they won't be overly busy.
That said... they seem high maintenance to me. Since you are new, you may have to establish yourself over time, but in the beginning of my hobbying I would call places, and make apt the same day. I still use that strategy for most non-agency AMP establishment. Any apt made more than a day away can be forgotten or pushed because they are never sure if you will keep it if you are not established.
If you're stuck on using them, text them 2 hours before but have a back up plan. If they respond oddly or not clearly enough, I would bail, and schedule an hour at any AMP you see has good reviews here.
This way, you will be more guaranteed your slot, and get your feet wet in the hobby.
Good luck.