[asterisk-users] What are the reasons for VoIP echo?

[Peder]

On 10/02/2009 08:36 AM, Martin wrote:
> if a user calling you hears echo of himself then it's the fault of
> your sip device/sip phone.
> The manufacturer must be using a cheap or an open source echo canceller
...
>
> try getting a different sip device made by some 'normal' company like
> polycom or linksys/cisco
>    
> You might like to know that a number of people have used the open source 
> OSLEC canceller to replace the rather broken one Linksys put in their
ATAs.

How would you do that for just those ATAs and not every phone in your
network?