Grandstream GXP2160 DTMF issue with 3CX Voicemail

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I am having an issue with one customer where DTMF tones are not being recognized by 3CX voicemail system. They will open mailbox and sometimes it recognizes the PIN on first try and sometimes it takes a couple of tries. Then when they press star to play messages sometimes it works sometimes it repeats the same message again. Any suggestions on this issue?