I managed to get it working by forcing the phone to use G.711 with 20 ms
and not 10 ms. But for the sake of understanding what exactly

 <param name="inbound-codec-negotiation" value="scrooge"/>
and
 <param name="rtp-autofix-timing" value="true"/>
does?

I saw a BUG in Jira the refers to this second SDP for Sonus
 http://jira.freeswitch.org/browse/FSRTP-8
I wonder if this is Sonus specific patch.

Unfortunately we do not have 16 oz Hammers here in Sweden... so i
recommend this alternative!
http://www.ikea.com/gb/en/catalog/products/70082653