Just for clarity Richard, Viewranger does not use a mobile signal, it uses the same GPS satellites as a dedicated GPS unit. In fact if you want your battery to last using Viewranger you need to turn your phone signal off, I use airplane mode. If you want to use buddy beacon then you do need a phone signal and will suffer battery rundown Which in a low signal area can be fast, whilst the phone is hunting for a signal.. But since a dedicated GPS (such as Etrex) cannot use phone networks then they are no use for the OPs notification requirement either.
I have suffered serious battery problems with my phone mainly due to my own stupidity, I didn't flick the phone into airline mode. I carry a 14w solar charger which is enough to keep a power pack charged up
I have read the above mentions of VR being off course at times, in three years minimum I have never experienced that EXCEPT in built up or forested areas. Strangely I found out why by reading the Garmin FAQs seemingly it can happen with them also in the same type of terrain.( In areas when you are walking in GPS shaded areas (valleys, gorges forests, densely wooded areas.etc)).
I am sure Sussamb will be able to enlighten us further as he is an expert on all things GPS.