Check compatibility with the personal alarm company. They ought to be fully briefed and prepared for this changeover and should be able to confirm not only compatibility with VoIP/analogue telephone adapters but also confirm operation in the event of a sustained power failure. In other words they may have digital models with battery backup and cellular connectivity as a primary/backup means of communication should the digital voice service become unavailable.
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If you don't mind the FritxBox router I changed to DV with Zen quite a few moths ago. No problem with porting the old telephone number.
Also the FritzBox has an app so you can use your Iphone like a cordless phone. It can use your iphone Contacts and also works on an Ipad as handsfree.