Drop calls problem

Hello People
I set up in Costa Rica a 2850 Dell Server with 4 GB RAM, wich is optimized in order to get a 100%-99,98% zttest result, I use trixbox, I set it up for 160 people, the phones I use are GrandStream 2000.
But the calls get dropped and also I have echo in a random way and in some numbers there is always echo, I know that grandStream phones have issues with echo, but I don’t understand why calls are getting dropped (the phones show an 503 error).
Any Idea to resolve this?

Another issue is when a call is made and it is in a ringind state, one can hear two tones, like one is from de line of a E1 and the other is made from asterisk, it is like there is a synchronization problem?