I know this may sound crazy, I have had the same issue when entering any commands while connected to a node, *81 or *3..... disconnects after 30 seconds,
Now the only " Solution " I have found is to add a second entry for the node, another 'Line' as they call it, and use that one, then it works just fine. I have checked & cross checked the settings on the phone, even if I have multiple servers, only the first entry on the IP phone has this issue (I have 4 separate lines/servers -Not Nodes) You could duplicate an entry with the same node, it's ridiculous but it is working.
The phone is a VVX 500 series, I have updated the firmware and tested this on two different phones.