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I have just acquired a new suresignal after my previous one blew, which was a replacement for another that blew similarily after 36 months. I've been onto live chat a few times and they have been able to solve a defective initial registration and then add my wife's number but I keep getting the above error when trying to add or maintain users. This has happened not only with this unit but with it's predecessors also.
Can someone please explain what the error is as it means nothing to me and offer the instructions to fix it.
The general advise is to deregister the Sure Signal, leave it for 24 hours before reregistering and then try adding the numbers.
As the Sure Signal gives 3G reception, making sure the phones you are adding are set to 3G can help.
Please see this link for further information: My Sure Signal is working but not with my phone. What can I do?