Good morning Graham,
Thanks for your message and being a part of the TPG Community.
For a VOIP service to work it is configured using the MAC address and Serial number of your TPG supplied modem as part of its protocol.
As all modems have a unique MAC and S/N it is not possible to have a spare modem setup for VOIP.
If you were to use your spare modem you would need to contact TPG to reconfigure your VOIP service to your spare modem at that time.
If you were to have a power outage or use a 3rd party modem you will lose your VOIP service
Due to the loss of Internet and VOIP services during power outages I would encourage you to obtain a mobile telephone to use during this time and/or get a medical alarm installed by a reputable company to assist you in times of ill health or an emergency.
I hope that advice helps.
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