ERROR: value out of range: underflow

Hi.

I have a simple function.
CREATE OR REPLACE FUNCTION myf_convert_phone18digits(
in_phone VARCHAR
) RETURNS BIGINT
-- IMMUTABLE
AS $$
DECLARE
t_extent_len BIGINT;
t_phone_18 BIGINT;
t_multiplier BIGINT;
BEGIN

IF in_phone IS NULL OR in_phone = '' THEN
RAISE EXCEPTION 'in_phone[%] IS NULL OR =''''!', in_phone;
END IF;

t_extent_len := 18 - length(in_phone);
t_multiplier := 10::BIGINT ^ t_extent_len::BIGINT; --<<< ERROR HERE
t_phone_18 := (in_phone)::BIGINT;
t_phone_18 := t_phone_18 * t_multiplier;

RETURN t_phone_18;

END;
$$ LANGUAGE plpgsql;

When I try it in pgAdmin, first time it show me error:
--
ERROR: value out of range: underflow
CONTEXT: PL/pgSQL function "myf_convert_phone18digits" line 12 at assignment
--
SECOND (and consequences) time it works and just show result 771506000000000000

I get same problem when I try to call query from web (php).
I can't do "second time" here, so web always fail with "ERROR: value
out of range: underflow"

What's the problem? What can I do with it?
--
antonvm