UCM6510 & E1 problems


I have installed a UCM 6510 and I have an E1 line for incoming and outgoing calls. In calls, users complain that they are not listening well. There are delays and sometimes the line falls. Sometimes there are also delays in internal calls. The telephone sets are GXP2135, GXP2160 & GXP2170.
Do you have any advice as I am new to IP telephony?

Best regards


excluding UCM could be the switches not suitable for Voip, does the Poe exist? of quality?
Is the network made correctly and certified?
The VoIP particular cares and knowledge to know in advance.
My advice is to start with an analysis of each factor that involves the operation of your infrastructure.

n.b.: excluding UCM I did not mean that it certainly works, everything could be.