3-way conference DTMF relay issue

We want to perform live customer demonstrations of an interactive survey system by having the system call the SNOM 360 phone we are using then bridging us, the customer and the survey system call together using the 3-way conference feature. We then want to allow the customer to respond to the survey using DTMF.

Everything works great except the DTMF part...from the SNOM. When we try the same setup from an XLite client it works flawlessly. Same result using a Linksys IP phone.

Any suggestions or ideas would be much appreciated.