In short, it means that the system cannot handle the requests fast enough…
If you don’t really use FreePBX other than for creating the Trunks, I suggest writing a complete custom dialplan.
There are a lot of parts in the FreePBX dialplan that are unnecessary for your use case, so in theory, if you don’t call all the AGIs that FreePBX uses then it’ll probably speed things up.
With that being said, 500 calls per day should not trigger these overloads with standard hardware resources. So I suggest you actually try to find out what is happening here.
There are a number of threads on this and the Asterisk Community Forums on this subject.