consecutive row count, MySQL

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drop table calls;

create table calls (phone_number varchar(100),system_outcome varchar(20), dt datetime);

insert into calls values 
('07777778999',       'Answered'        ,'2016-12-18 17:15'),
('07777778123',       'No Answer'       ,'2016-12-18 18:10'),
('07777778999',       'No Answer'       ,'2016-12-19 19:30'),
('07777778999',       'No Answer'       ,'2016-12-19 12:15'),
('07777778999',       'No Answer'       ,'2016-12-19 13:15'),
('07777778999',       'No Answer'       ,'2016-12-20 11:15'),
('07777778124',       'No Answer'       ,'2016-12-20 9:15'),
('07777778124',       'Answered'       ,'2016-12-21 9:15'),
('07777778128',       'Answered'        ,'2016-12-20 17:15'),
('07777778074',       'Answered'        ,'2016-12-20 17:15'),
('07777778074',       'No Answer'        ,'2016-12-21 19:25'),
('07777778074',       'No Answer'        ,'2016-12-21 21:25'),
('07777778074',       'Answered'        ,'2016-12-22 00:25'),
('07777778074',       'No Answer'        ,'2016-12-22 10:05'),
('07777778999',       'No Answer'       ,'2016-12-21 13:15'),
('07777778999',       'No Answer'       ,'2016-12-22 11:15')
;

select phone_number from (
select c.*,
@prev_outcome:=@cur_outcome,
@cur_outcome:=system_outcome,
@prev_pnum:=@cur_pnum,
@cur_pnum:=phone_number,
case when @cur_pnum = @prev_pnum and @prev_outcome <> @cur_outcome then @rn:=@rn+1
     when @cur_pnum = @prev_pnum and @prev_outcome = @cur_outcome then @rn:=@rn
else @rn:=1 end as rank
from calls c, 
(select @rn:=0,@prev_pnum:='',@cur_pnum:='',@prev_outcome:='',@cur_outcome:='') r
order by phone_number,dt
) x
 where system_outcome='No Answer'
group by phone_number  ,rank
having count(*) >= 6 -- change this per your conditions


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