This seem sufficiently answered, but a few thoughts.

First, internet and VOIP are immediately backed up by UPS at my house. These tend to last several hours for me. You might plug your 8 hours backed up device into a UPS and stretch it to near the 24 hours you want.

Second, you can buy rechargeable D cells. So you could buy the 24 hour backup system and periodically top up the D cells. In fact, you could store more D cells and make it last even longer.

This is all pointless if the equipment on the other end of the line is down. That's what tends to happen where I live. Everything (cable and cell systems) has backup power for a few hours but eventually fails. Most service provider sites are rigged for generators but don't have one onsite, so unless one is brought in, the system stays down. Last time we lost power my cable modem was blinking but there was no one on the other end.

To complete my answer your actual question, I have a generator that recharges the UPS(es) and I can keep things going for a few days if needed. For a while I had starlink internet, which would in theory stay up during an outage. I switched back to cable, unfortunately.

Final comment, I'm getting solar and batteries. With that the house should be able to run quite a while w/o grid power.