Is voltage an electromagnetic signal?
It's a field rather than a wave, where EM waves are changes in the local EM fields. The classic diagram shows the relation of a wave to the two types of field, electrical and magnetic.

Wherever there is a voltage between two points, there will be an electrical field between those two points. If there is a current flowing along a wire, there will be a magnetic field around the wire.

Some devices rely on electric fields for their operation, notably the field-effect transistor, but also the thermionic valve where the field controls electron flow between two plates in a vacuum.