What kind of phone are you using? Is it a US phone? Have you tried a different phone?
With "DTMFRxMode" set to hardware (default), I have personally tried many different phones (US, European, Australia), and I haven't seen any problem with OBi110, either 2384 or earlier version 2286 ... Digits are always recognized well.
Also, does the voice sound OK on this phone? how does the other party feel voice quality?