It is just a feature of the “phone” keyboard, it has little or no use for the specific application.
It is something with pausing, waiting for dial tone and whatnot. E.g. if you want to program an inside extension number in some oldschool switchboard or dial your way through a phone tree by default.
Nitpick, but potentially important: it should be “prefix to block” - “start of range” is something different.
Also you putting the + outside the parenthesis suggests a possible misunderstanding. The number to block contains the plus - it is not some control character like the ^.