such as this small battery backup unit from Eaton which costs under £50
I see that particular model only has a rated capacity of 2200mAh, which assuming 3.7V lithium cells is 8.1Wh. It might power a router and ONT for for 30 to 60 minutes if you're lucky; that will go down as the cells age.
If you have a decent mobile signal where you live, then a mobile phone may be a better bet for emergency calls.
Got one here on test...
Currently running a CRS-305-1G-4S. Been running 1 hour and 23 minutes so far and looks like there is plenty of capacity left. I'll run it till it dies and let you know.
Also have a spare N330A-IP DECT base kicking around, so will test on that too; talk time and standby time.
I don't do BT, so cant check with a SH2 unfortunately.
As above I ran the test on a little Mikrotik CRS-305-1G-4S using the Eaton mini UPS to hold it up and got a fairly respectable
3 hours 17 minutes and 51 seconds before the Eaton gave up.
I will have to measure the exact power consumption of the MikroTik when its at idle - but from what I've read it
here should be 10 watts in idle, with a max. observed of 13 watts.
Unfortunately I've not been able to test the Gigaset N300A IP DECT base with this UPS as none of the supplied DC barrel tip will fit the N300's power input port. I will have to either splice the lead from the plug pack or make an small adapter lead up with a female to male barel adapter arrangement. So for now I cant say how long it would hold up the N300A.
At some point I may rig it up to an Openreach 1+1 ONT and give that a try.