For incoming calls on some extensions, calls go directly to the voicemail.
The Grandstream phone is registered and DND button is OK.
The extension is registered and I can call out
We have recently implemented an IVR and the Freepbx version is 220.127.116.11
Your help would be greatly appreciated to solve this problem.
Post your Asterisk CLI output at the moment of the call please
You need to ask the people that wrote your dialplan! As you mention FreePBX, you may be lucky at https://community.freepbx.org/ but you are almost certain not to find them here.
If you are not using FreePBX, the same applies, except we would have no idea how to find and contact them.