Can phone apps be used when there is no signal to track elevation, distance, speed?
Yeah, as long as your phone has a GPS (which is most likely any Android, iPhone) it can act like a GPS unit and track that sort of stuff. They can really do just about anything a Garmin can do, with the right app. The downsides of using a phone for this is that it's not as tough as like, a Garmin (Garmin: waterproof! Phone: no.) and the battery gets drained pretty f'n fast - and then: no phone. I use my phone to track <6 hr bike rides and things like that. Anything else, and it makes a ton more sense to actually use a dedicated GPS unit. I've cheated with my phone, and use an outside battery pack to charge my phone as it's tracking, but it's a pain, and the micro usb cable likes to break. Often.