Cheers for your help AceJavelin,
We have got the systems sharing data and get no SDS errors, we do however get this error when looking into the Maintenance Logs.
Log Type Software
Log Number 2046
Severity Warning
Date 2016/Apr/20
Time 17:39:48
Source Call Control - Software
Description
SWERR range_check_error, OFFSET 2:$15EC, PC: $5D182A LOG_ID: xnetlh#98, PROCEDURE: xnetlh_handle_rmsg_open_ind, SEVERITY: cp_inform PID: ($2, $60C) Xnet Rejected open for ICP: 001 Received Invalid IP: 192.168.121.254
Module Main
File Name and Line Number log_outputs_util_en.c;1227
I am now suspecting the routers between sites to be modifying the packets in some way but I will confirm with the IT company tomorrow. As VisualSpirit says, there are no firewalls, just routers on an MPLS.