Grandstream Networks GXP 1620 User Manual page 8

Hover cursor over [Accounts]. Select [Account 1] and click [Audio Settings].
7.
Send DTMF: Check [via RTP].
*If you check [DTMF Inband] for [DTMF mode] on your Unique Management Page on
our site, Check [in-audio].
Once done click [Save and Apply]