The default feature code for paging is 7. The 1 in 71 is for page zone 1, the 2 in 72, is for page zone 2, etc.

In each of these page zones, you can have any combination of keysets (phones), CO trunk ports, or the page port on the phone system.

There is also a page zone 0 so it could be feasible that '70' would be an all page.