on the MEL controller, do a "LOCATE EXT " on the SYD extension.
Does it recognise it as a "Remote Directory number"?
On the Remote Directory Numbers form, look for the SYD extension - check the CEID.
On the Cluster Elements form, check against this CEID and see which CEID digits this has
On the ARS Digits Dialed form, find the CEID digits and check which route this points to
On the ARS Routes form, find the route and check the Digits Mod is correct/the same as for all other similar routes
On your ICP/PBX form, check your CEID for SYD, on the IP Network tab what IP address is there.
On Maintenance Commands, ping the IP address in that last form
Check MEL & SYD for SDS errors.