Echo on handsets (but not in recordings?)

Hi All,
We have just deployed a FreePBX server with Yealink t28 phones and have noticed an intermittent issue with echoes on some calls.

Callers on our end get massive echoes of themselves, often louder than the person they are speaking to with quite a large delay.
The issue also appears to affect softphones (microsip), both internal and external calls.

I have checked the recordings and they a crystal clear, no echoes or distortion of any kind.

I have disabled all the voice enhancements on the handsets themselves, no change.
I am not yet running any additional echo cancelling modules.

Any ideas on what I have missed?

FreePBX 2.11
Asterisk 11
2x OpenVOX B100E
1x OpenVOX A400P FXS
Yealink T28p
New Zealand

Can you attempt a call from a sip trunk provider like flowroute.com (or something local to NZ) to see if the SIP vs POTS makes a difference?

The only time I’ve ever run into an echo problem is when I’m using a T1/PRI card (against my better judgement) or I’m using a POTS adapter (network or otherwise).

Actually appears the echo has mostly disappeared now…

Have setup a 2talk.co.nz connection over IAX, will test over that if the issue decides to show itself again.