Able to connect only one extension number to PJ_SIP server

I have set up manhy extensions over my local server, but when I try to set up the same number of extensions over a remote server, I am facing problems.
Only the latest extension remains connected. The rest of the extensions throw a Request Timeout error (408).
I am not sure what the problem is here, if it works on a local server why does it not work on a remote server ?