Cbeyond sip-proxy sipconnect

There are a few posts about phone calls being drop after 10 minutes (and a few seconds) systematically, none address this issue.
I'm using Asterisk 11.4.0 FPBX-2.8.1(11.4.0).
After hours with Cbeyond level-2 support, they told me that their system (Brocade) send a test message (or something like that) every ten minutes and expect a reply from Asterisk.
The request is coming from the 'sip-proxy' server, but our reply (that indeed exists) is being sent to the 'sipconnect' server. (?)
I'm lost. Everything else works fine: incoming calls are routed and processed without problem (any length). Outgoing calls using other (non Cbeyond) vendors also works fine (Callcentric, Skype and using SPA3102 boxes)
host=sipconnect.XXX0.cbeyond.net fromdomain=sipconnect.XXX0.cbeyond.net outboundproxy=sip-proxy.XXX0.cbeyond.net

Help is highly appreciated,