This was going to be something explained in a 'the plot begins' post when the game started but it's hardly worth keeping secret and since you asked;

The Network can only be accessed when a threat to Earth is present, and in the same vicinity of the area the characters can travel and retain their Echoes. So as far as the characters know, there's no such thing as the Network and the numbers are just something stuck in their head. At the plot's start point they will a pseudo-Echo which reminds them of their numbers very insistently, in case they had their first Echo a while ago and stopped thinking about the weird numbers they can't forget.

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