Call quality fine, but sounds like person is speaking through a towel

Hi All,

We’ve just got our PBX all setup and it’s running really well, but the only thing is, the extension-to-extension calls sound like the other party is very muffled; like they’re talking through a towel, or a mouth full of cotton.

I’ve tried swapping the order of extensions; but this doesn’t seem to have helped (rebooted after swapping codec order).

All ext-to-ext calls are using Digium D50 phones.

The PBX is Asterisk 11 with FreePBX 2.11.

Any pointers in the right direction would be most welcome.