util.service('Util',[dependencies... {

  //SEARCH FUNCTION
  this.buscar = function (addr,options) {
    Restangular.setFullResponse(true);
    var result = Restangular.all(addr).getList(options);
    result.then(function(response){ //success },
    function(error){

      //THE PROBLEM OCCURS HERE
      this.retrieveToken();

    });
  };

  //RETRIEVE TOKEN IF EXPIRED
  this.retrieveToken = function (){ //code... };
}]);