Help with trunk setup on legacy pbx

I’m setting up FreePBX alongside our old legacy analogue pbx which is a Siemens/Norton MCX (EMS 601). This is t allow a phased move to FreePBX of about 700 extensions.
I have dialling from FreePBX to extensions on the old pbx working well using an FXO box connected to extension lines on the old system. For dialling from the old system to FreePBX I’m using spare trunk lines on the old PBX connected to a FXS box. This is almost working, in that you can dial extension on FreePBX from th e old system but there is a 10 second delay in the call being connected. I’ve narrowed this problem down and it’s because the old system cant detect the dial tone generated by the FXS box, and it waits for a 10 second delay before then dialling the call. I have tried all the obvious dial tone frequencies/levels etc but it’s never detected. I’m not sure that the trunk card can even detect this having looked at other working trunks on the system.
Looking at other trunks on the old system this delay can be set down to 1 second which means they don’t wait very long for a dial tone but just dial, but I have no information on how to change this timeout value, and nothing I have tried changes it for my new trunk.
Bit of a long shot but does anyone here know anything about the Siemens MCX system and know how to set the “Dial Tone Wait Timeout” for a trunk?