Add a “no cell network available” message

[DodoLeDev]

Checklist

• [x] I am using the latest version - 1.4.6
• [x] I checked, but didn't find any duplicates (open OR closed) of this issue in the repo.
• [x] This issue contains only one feature request. I will open one issue for every feature I want to request.

Describe the feature you want

When a call is interrupted because of a cell network outage, I suggest that Koler shows an appropriate error message in order to clarify the situation.

Is your feature request related to a problem? Please describe it

When my call is interrupted because of a cell network outage, nothing but me can help me understand why my call has been unexpectedly stopped!

Additional context

How will you/everyone benefit from this feature?

I think an error message like that can help people clarify why there was a problem with their call, instead of blaming the beta status of the app!