Unable to bring up dial-pad in call (URGENT!)

[helios.comms]

I've just upgraded my client's four Teams phones to the latest firmware: CP960-73.15.0.45.rom

However, they have reported that once they have established a call, there is no way to get the dialler back so they can enter enter (for example) a conference code to join a meeting.

This is the screen: https://i.imgur.com/Y1HzgTd.png

How do they bring up the dial-pad during a call?? This means the phones are currently nearly useless as they are used primarily for conference calls.

My immediate thought was to downgrade the firmware to an older version CP960-73.15.0.38.rom but when I try this, it fails to upgrade with this error: https://i.imgur.com/tc8Qdsh.png