The statement "The phone should be restarted" is not a hypothesis of a cause. It is more of a suggestion or a recommendation for addressing the problem of the smartphone having no signal, rather than a testable statement predicting a causal relationship.
Hypotheses of a cause would typically involve statements that suggest a specific cause leading to a specific effect. In this case, restarting the phone may be a way to resolve the issue, but it does not directly identify any underlying cause for the lack of signal.