Try resetting the phone’s network settings. This procedure does not erase any user data, though Wi-Fi passwords will have to be re-entered and Bluetooth devices will have to be reconnected (if applicable). The reset network settings feature is usually found in the backup and reset settings menus of phones.

If this does not resolve the issue and you suspect there are network issues in your area, you can submit an outage report. 

To add on to rikkster, I would suggest you to enable wifi calling if you are using a compatible device. That way, you can still make calls and send or receive text messages when you have little or no network coverage. https://www.koodomobile.com/en/help/wifi-calling

Are the problems limited to a particular location?

Could be a local dead zone where reception is poor or non-existent. If your phone has an “about phone” option in settings, that should tell you how strong the local signal is. Ignore any bars shown on the main screen.

@BobTheElectrician furthering this thought, might also possibly be some recent spectrum refarming in the area?

@Amey Simkins what devices are your daughters using, out of curiosity?