On a different VoIP provider:
I had unreliable IVR successes with the Linksys ATAs. Had two. Some IVRs would work, others would not. The ATA seemed to distort the DTMF tones. Even the VoIP provider's voice mail had trouble, as did my drug store's prescription IVR and my employer's big-league voice mail.

Using the SIP signaling where the ATA detects DTMF tones and replaces tones with SIP messages (is that "OUT OF BAND"?) did not work for me. The VoIP provider (ViaTalk) actually blamed the Avaya PBX I was calling to for being at fault.

So far, no such issues with whatever default DTMF setting VoIPo has with the Grandstream.

But this was for me issuing DTMFs in accessing IVRs. Female voices can be mistaken for DTMFs by some crummy ATAs. That's a really lousy product, since the DTMF tone-pairs were carefully chosen long ago to preclude this.