If you need to coordinate with someone who isn't sitting by a radio, you need to call them (including the local govt, your own civilian personnel, etc.). You might not be issued a radio at all, so a phone is what you have.

Commercial telecommunications are an important part of any military worldwide, when they are available - so you use them until someone actually starts DFing you accurately, and then even after that, it's a risk to be managed and certainly not black & white.