Modem being switched off / ofonoctl indicates registered but dialer unable to make calls / spacebar error messages

On testing branch, recently the modem ends up in an unpowered state. Sometimes ofonoctl says modem is registered with the network but Dialer says Unable to make a call and Spacebar displays error messages when trying to read a text message/sms.

Issuing ofonoctl online does not always work. Rebooting typically solves the issues though there have been times when the phone rebooted and the modem was still unpowered.

The problem seems to occur after several suspends or after some time in suspend mode, unless perhaps the issue is when the phone is resumed.