Remote autentication

I have a problem with ocillation remote conection.
Some times register normally and some times no.
I made some tests put down the firewall (iptables and router) but still continued problem.
When I capture packets with tcpdump I saw that the pbx do not response when not connect. So the error is timeout. But sometimes I tryied immediately after and it works.
There is some rule of sip registrations number? Some limit maybe?
Someone can help me?
Thank you!

You have provided absolutely no information on your system which makes it hard to assist.

Do you have the networks of the remote extensions in the local nets?

Hi Alan,
I don’t have any networks external like local nets. I don’t understand very well what you want to say.
About informations, it is about versions ?