It might be caused by ATA setup for DTMF handling

Are you using ATA for Gizmo5 setup? It might be caused by ATA setup for DTMF handling.

Both scenarios require you to enter PIN, which is DTMF related. If your ATA doesn't send out the keys you enter correctly, it will cause issue on the far end, T-Mobile voicemail system or conference.

You can try with Gizmo5 soft-phone(run on your PC) for the same scenarios. It that works, it's your ATA DTMF setting for Gizmo5.