Jenny is driving across the country. At times, there is no signal to her mobile telephone or pager. What might be the reason for this?

Out of coverage zone

Explanation:

Assuming Jenny has charged her devices, since she can see there is no signal, the most logical reason is that she's out of the coverage area.

Cellular phone service is assured through transmission towers. These towers are very numerous in urban areas (cities) but get fewer and fewer as you move away from the cities. Why? Because in some areas there's not enough need for the service to justify the cost of installing the towers required for that. So, Jenny might just be in a very remote location.

Coverage could also be limited due to natural geography, like mountains, that block the signals.