The corresponding "atomikky" predicate, however, is called atom_number/2

Note that unlike other predicates, the String must indeed be a string, an atom is not accepted (at least for now). However, list of characters (atoms of length 1) and list of unicode code points are allowed:

:- begin_tests(number_string).

test("usual case", true(S == "123456")) :-
   number_string(123456,S).
   
test("usual case, with leading 0s", true(S == "123456")) :-
   number_string(0000123456,S).
   
test("parsing integer", true(N == 123456)) :-
   number_string(N,"123456").
   
test("parsing integer with leading 0s", true(N == 123456)) :-
   number_string(N,"0000123456").
   
test("no leading whitespace in string", fail) :-
   number_string(_,"    123456").
   
test("no trailing whitespace in string", fail) :-
   number_string(_,"123456     ").
   
test("fails if string-to-parse is not a number (rather than throwing) #1", fail) :-
   number_string(_,"NaN").
   
test("fails if string-to-parse is not a number (rather than throwing) #2", fail) :-
   number_string(_,"Hello, World!").

test("direct verification") :-
   number_string(123456,"123456").

% ---
% UNEXPECTED: "Type error: `list' expected, found `'123456'' (an atom)"
% ---

test("weird exception 1", true(N == 123456)) :-
   number_string(N,'123456').

% ---
% UNEXPECTED: "Type error: `list' expected, found `'0000123456'' (an atom)"
% ---

test("weird exception 2", true(N == 123456)) :-
   number_string(N,'0000123456').

% ---
% The above is explained by the fact that number_string/2 can actually process
% lists of unicode code points or characters to represent valid numbers, too:
% ---

test("Bad code points", fail) :-
   number_string(_,[1,2,3]).

test("Good list of characters", true(N == 123)) :-
   number_string(N,['1','2','3']).

test("A conversion too far: list of stringy characters", [error(type_error(_,_))]) :-
   number_string(_,["1","2","3"]).

test("Good list of code points #1", true(N == 123)) :-
   number_string(N,[0'1, 0'2, 0'3]).

test("Good list of code points #2", true(N == 123)) :-
   number_string(N,[0'0, 0'0, 0'1, 0'2, 0'3]).

:- end_tests(number_string).