Turn on led by phone and phone contacts
So, retrieveCallingNumber is documented in here. It "retrieves the calling number, and stores it" in a character array passed as second argument.

The simplest solution to check if this number is among the allowed ones is to construct a string from this and compare it against a predefined string for equality. (If you have more than one allowed number, repeat for all.)

Something like this:

vcs.retrieveCallingNumber(numtel, 20);
String s = String(numtel);
if (s == "allowed number goes here") {
    // toggle LED etc.
}