In the UK,
emergency calls to
999 are always
toll free. Even telephones which have been disabled in some way, often accept
999 calls.
999 applies to all telephones, even
mobile phones.
The number 999 was chosen because it is easy for a blind person to dial on a rotary telephone dial, but it is unlikely that a child would accidentally dial it (unlike 111). Of course, on a modern telephone with buttons, 999 is equally likely for young random hands to dial as 111.