Goodbye from heads of Christian Union on the phone (3,3)
I believe the answer is:
see you
'goodbye' is the definition.
(I know that goodbye can be written as see you)
'heads of christian union on the phone' is the wordplay.
'heads of christian' becomes 'see' (I can't justify this - if you can you should believe this answer much more).
'union' becomes 'U' (common abbreviation as in Trades Union Congress).
'on the phone' indicates a 'sounds like' (homophone) clue.
'u' sounds like 'you'.
'see'+'you'='SEE YOU'
'from' acts as a link.
Can you help me to learn more?
(Other definitions for see you that I've seen before include "poker-playing expression?" , "For so long" , "Later we'll meet" , "Casual and informal goodbye" , "Informal kind of au revoir - with visual intent" .)