Asterisk Gui was a product that Digium was writing on their own.

Freepbx is a gui that a community wrote. Digium now uses Freepbx on their AsteriskNow product. They have switchvox which has its own gui for people that pay.

The issue with Gui's are the macros that are written do not allow you to really make changes through CLI to the conf files.

I would not really suspect the Aastra phones, I would suspect the network, echo comes from cheap handsets, or even the cards. We need more info, what type of TDM hardware is there? Are they pots lines? Or are they using an ATA to convert lines to SIP? Or is it a PRI/T1? What version of Asterisk are they running? Zaptel or Dahdi? Which echo canceller are they using the standard one MGCP or Oslec? Oslec is a better choice.... I had a customer with horrible echo I installed Oslec and cleared it up right away.

Also their bandwith to the internet is a mute point, unless they are sending calls over it....