I have setup a Grandstream GXP-2000 successfully, however when using an automated system while on a call, the tones are not sent over the phone system.
It should be the DTMF issue.
Try to change the DTMF the type:
1. Login your GXP2000 via web admin page
2. Goto Tab "ACCOUNT 1"
3. Select "via RTP(RFC2833)" at "Send DTMF". If it still not works, try the other one "in-audio".