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The phone line(s) are through the modem. If you have a power failure or there is a server problem, then no phone line.
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Just some comments on the above .
All Telecom providers in the world that provide VoIP telephony from DSL or Cable modems have the above 'disadvantage'.
Among them CYTA Hellas who provides VoIP telephony at different areas of Greece.
Nobody provides a UPS, so you can buy a cheap UPS and do your job.
Probably you also have a mobile phone so anybody looking for you probably will also try at your mobile.
