Better option… just strip all non-digit characters on input (except ‘x’ and leading ‘+’ signs), taking care because of the British tendency to write numbers in the non-standard form +44 (0) ...
when asked to use the international prefix (in that specific case, you should discard the (0)
entirely).
Then, you end up with values like:
12345678901
12345678901x1234
345678901x1234
12344678901
12345678901
12345678901
12345678901
+4112345678
+441234567890
Then when you display, reformat to your hearts content. e.g.
1 (234) 567-8901
1 (234) 567-8901 x1234