There will be two files created, one is the number.txt and name.txt
If any of these files are missing this pop up will come.
To avoid that you have to save any fax number. This will automatically create the files in the necessary format
Yes, you will have to setup the account giving the username and password as per the extension you created in the Elastix Server, you will need to create SIP extensions in the server to do the same.
The SIP proxy on the grandstream device should be given as the Elastix Server IP. This link...
So you mean to say that, these phones can communicate with each other using Elastix server. If this is true, you will just need to create a trunk to the PSTN as mentioned in the Elastix without Tears and create a route which will be used by these phones to make calls out through the PSTN
As mentioned by Megabyte, it is a broadcast that is being send. The phones have to be in the same network to be detected by Endpoint manager. There is another post somewhere in the Elastix forum which gives a complete explanation on how the Endpoint manager works in Elastix