"Man må slå av og slå på telefonen."
Translation:One has to turn off and turn on the telephone.
Could this sentence be translated as "you have to restart the phone."
I was thinking the same thing. Restarting a phone is turning it off and turning it back on again. Is there a separate word for "restart" or what?
You can use either "restart" or "omstart" for that (they work as nouns and verbs), "å starte på nytt" is also quite common in addition to "å slå av og på (igjen)".
In this context, it means "off", but often prepositions can form part of phrasal verbs that do not translate directly, which is why they end up with a myriad possible translations.