Basic "phonebooth" kit with the phone (GXP-1630) going to the Mikrotik SXTsq 5. Front right corner has two RJ45 panel mounts. 12V/5V step-down for the phone. 10Ah Dakota Lithium battery. Apache 2800 case.
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I haven't tested yet how long it would run. Much will depend on the use profile, standby vs. in-use.
The wallwart for the phone is 5V/0.6A. The stepdown converter claims over 95% max. conversion efficiency.
https://help.mikrotik.com/docs/display/UM/SXTsq-series
In that case, perhaps it would be a good idea to have a solar panel - something around 25 watts @ 12VDC to keep the battery topped off.