Need help with SQL query and finding NULL array_agg

Hello ppl,

I have a problem with one sql query, can some one help me. My query is:

SELECT array_agg(month) AS month, array_agg(status) AS status,
array_agg(service) AS service, case when array_upper(array_agg(phone),
1) is not null THEN array_agg(phone) else array_agg(mobile) END FROM
bills WHERE status > 1 GROUP BY mobile

I try with simple query to identify did array_agg(phone) is empty or
null and if is it to return me mobile field, if not empty to return me
phone. Mobile field exist always, but phone may exists for that mobile
may not exists.
One mobile can have few services like:

mob service phone
1321543434 64
1321543434 66 13255555

I try few thing but sql only return me records that phone is not empty,
but I need them both.

Any one has ideas what I can do ?