I think this is very good tip for people who are going out into places of low habitation where mobile telephone signal coverage is likely to be poor. The idea also reduces the risk of you running out of battery power at the time you need it most, in an emergency.
The idea is within the party to carry a small, fully charged, "emergency" telephone which is left turned off so that it is fully charged if it is needed. The second part of the plan is to have with it a selection of pay-as-you-go SIM cards for the phone so that if you find you have no network coverage you can try the other SIM cards and hopefully at least one of them will provide some coverage.
Certainly well worth the effort if a group is out in the middle of nowhere. This approach would probably have saved a guy I know a long and painful crawl when out on his own hiking recently when he turned his ankle and couldn't walk. He was in the Scottish Borders, with no mobile phone signal on the network he was using.