SES Server and loss of audio

[BolivianFast]

I have a problem with the SIP phones of customer registered in the SES Server.
Random the audio is loss; the command list tra tac XXXX show this screen:

Good Call

16:52:53 Calling party station 3997 cid 0x3016
16:52:53 Calling Number & Name 3997 Pruebas AES
16:52:53 active station 3997 cid 0x3016
16:52:53 G711MU ssff ps:20 rn:1/1 73.20.200.5:2246 172.31.90.10:55680
16:52:53 xoip: fax:Relay modemff tty:US 172.31.90.10:55680 uid:0x50003
16:52:53 dial 3998
16:52:53 ring station 3998 cid 0x3016
16:52:53 G711MU ssff ps:20 rn:1/1 73.20.50.150:2048 172.31.90.10:55684

Fail Call

time data

16:21:03 Calling party station 3997 cid 0x2b2e
16:21:03 Calling Number & Name 3997 Pruebas AES
16:21:03 dial 3998
16:21:03 ring station 3998 cid 0x2b2e
16:21:03 G711MU ssff ps:20 rn:1/1 73.20.50.150:2048 172.31.90.10:48588
16:21:03 xoip: fax:Relay modemff tty:US 172.31.90.10:48588 uid:0x8b73
16:21:03 dial 3998
16:21:03 seize trunk-group 1 member 15 cid 0x2b2f
16:21:03 Calling Number & Name NO-CPNumber NO-CPName
16:21:03 Proceed trunk-group 1 member 15 cid 0x2b2f

In the second call, the audio is loss.
I need your help, thanks.