BEGIN;
ALTER TABLE phoneline add column voicemailnumbernew varchar(16);
ALTER TABLE phoneline ALTER column voicemailnumbernew set default '';
UPDATE phoneline set voicemailnumbernew = voicemailnumber;
UPDATE phoneline set voicemailnumbernew = '' where voicemailnumbernew IS NULL;
ALTER TABLE phoneline ALTER column voicemailnumbernew set not null;
ALTER TABLE phoneline RENAME voicemailnumber to voicemailnumberold;
ALTER TABLE phoneline RENAME voicemailnumbernew to voicemailnumber;
ALTER TABLE phoneline DROP COLUMN voicemailnumberold;
COMMIT;
