It appears that may be an old version. It's most like a setting, or combination of settings. Possibly the VoIP calling switch is turned off and/or the phones data/wifi is turned off and there's not a signal.
I've taken a look at your case, and I think we'll need to gather some more specifics.
I'll make this post a private conversation so you can post more details. Please provide the following:
1. For the devices you've upgraded please confirm the device model and version of android OS,
2. Please confirm which version of the RingCentral App they are running.
3. Please confirm what phone number/ext the users with the problem are logging in with.
I'll add this information to your case if you provide it here please.