You can only use one method, local interface or OBiTalk to configure your OBi.
If you use the local interface to configure your OBi you must disable OBiTalk auto provisioning, otherwise OBiTalk will overlay any changes you make.
System Management -> Auto Provisioning
Under OBiTalk Provisioning
Method: Disabled
You won't be able to make changes from the portal with provisioning disabled.
OR
Use OBi Expert to make the DTMF change.