DTMF talkoff that you hear is being triggered by the remote party. It's more likely to occur with a female voice; your situation would be typical, if you are male and your contact is female.
The OBi has three settings that might help, though all involve some trade-offs.
First, try setting Physical Interfaces -> PHONE Port -> DTMFPlaybackLevel to -90, which should reduce the volume of incoming DTMF to nearly inaudible. This solution won't work if you have a device connected to the OBi, such as an answering machine, that you need to remotely control with DTMF. Also, it's not a true fix, because the beep will be replaced with (almost) silence, so there will still be a small gap in the incoming voice. If this setting doesn't change anything, the problem is unrelated to the OBi (it's at GV, or with the remote party's provider or equipment).
The other options are setting ITSP Profile A -> General -> DTMFMethod to InBand, and/or setting Codecs -> Codec Profile A -> Telephone Event -> Enable to (unchecked). These settings may adversely affect reliability of outbound DTMF, e.g. if you need to enter an account or credit card number when connected to an IVR system.
