If you are using a handheld satellite phone with the standard stick antenna it is very unlikely it will work inside a car. The antenna must have line-of-site to one of the satellites the phone uses so a metal roof would block this signal.
To solve this issue an external antenna is required. Magnetic antennas are provided with Iridium phones as a standard accessory which simply can be placed on top of your vehicle and the antenna cord can simply be attached to the back of the phone. Globalstar sells a car kit where the phone sits in a cradle attaching it to the vehicles power and an external magnetic antenna giving the phone line-of-site to one of the Globalstar satellites.Iridium, Globalstar and the Inmarsat IsatPhonePro all use omni-directional antennas, meaning you are not required to aim the antenna toward the satellite. It works more like a radio antenna that does not need to be pointed any direction, but it does need an 80% view of the sky to give good service. Because the antennas are omni-directional you would not need to be stationary when placing or receiving a call. Moving down the road would not disrupt your satellite phone service. Placing an antenna on the dash of the vehicle may work, but you might have a higher occurrence of dropped calls and periods of no service, because the antenna does not have an 80% view of the sky.
For larger emergency vehicles Fixed Site Satellite phones are available with stick and low profile magnetic antennas providing line-of-site to a satellite. Advantages of fixed site units most allow standard off the shelf home phones to be used providing a dial tone just like you have at home. These units provide a single line analog service that could be used with cordless phones. Each Fixed Site Satellite phone provides one phone line so if multiple phone lines are required you would need a Fixed Site Satellite phone for each line required. Excluding a few of the Inmarsat services and satellite phones most Fixed Site units have omni-directional antennas that do not need to be stationary to provide service. Like the Iridium, Globalstar and Inmarsat IsatPhonePro you can be rolling down the road and still have satellite phone service.