If I dial an unavailable endpoint, Asterisk throws an error. On the other hand, I have tested that endpoint state may not report itself as available at the time someones is calling it, and calls may be missed. How must I dial an internal endpoint without such errors?
What is the console output? What do you mean that it may not report itself as available?
I call a number, Asterisk says it’s unavailable, but in a 1 or 2 seconds it becomes available.
That doesn’t give enough information. I’d focus on understanding why it is unavailable, not trying to ignore that. If you really do want to ignore it then you can disable qualify if it is a SIP extension.