This tool produces phone numbers based on number ranges that are excluded for now (reserved for future use) in the Swedish PTS regulations.
Phone numbers matching 0100nnnnnn or 0109nnnnnn are within this reserved range, meaning they cannot (yet) exist as phone numbers in the real world, making these perfect for testing use.
The way the phone system work is you use a specific phone operator services. Sometimes you want to use another operator than the default one associated with your services, then you may choose a specific one. For example maybe you know of a deal from a provider with cheap long distance calls and want to use them rather than your own provider. Then you start the number with 95 and an operatorprefix (five digits).
One good thing about this is there is a test network that also has 95 as their operator prefix (easy to remember since it's 95 to also pick a specific operator).
This means there is a fail safe way of not routing your calls to any actual real world phone.
Mor information about the PTS rules can be found here .