My salary was halved due to virus. Should I work have the time and be on the line for the whole day? Or unreachable for the other hours also?

Does your contract allow for this?

That sucks man.. A friend of mine got a mail telling him to lower his hourly rate by 25% "or else..". Weird times

Unreachable. On-call duty would have to be paid extra. The only thing an employer gets for free should be a firm "no".

Anyone who is a contractor runs the risk of having their contract terminated. Depends on the terms, of course, but typically much easier than getting rid of an employee.

I'm happy I live in a country where I can't be fired unless a labour court judge agrees.

It depends on where you live.

How about slow down the work to half the speed?

@gronostaj technically my contract allows me to not work at all and still get paid, since the company entered "idle-time", just to reduce all the salaries and receive benefits from the state.

@HitWRight Then this isn't on-call duty. If the employer doesn't like that, he shouldn't have requested government aid due to low work load.

However, he can of course schedule the working hours as he likes - either to have people working simultaneously for cooperation, or in dissimilar shifts to always have someone available.