If you use your mortars in direct fire mode they just have to have visual contact with the target.
If you want to use them in indirect fire mode they have to be in communication with the unit doing the spotting. If the spotter is within audible or visual range of the mortar he will be able to call for fire. If not, he has to be in radio communication with the mortar. If the spotter (whether an actual forward observer or regular officer) has a radio then you need a radio near the mortar. This can either be another unit with a radio or a vehicle with a radio. Once the two units are in radio communication the spotter can call for fire from anywhere on the map.