Flaw restricting ring, Irishman set up telephone facility (9)

'telephone facility' is the definition.
(voicemail is available on a telephone)

'flaw restricting ring irishman set up' is the wordplay.
'flaw' becomes 'vice' (vice can mean a failing or flaw).
'restricting' means one lot of letters goes inside another.
'ring' becomes 'o' (letter resembling a ring).
'irishman' becomes 'liam' (common Irish name).
'set up' says the letters should be written in reverse (in a down clue, letters go up).
'liam' reversed gives 'mail'.
'vice' placed around 'o' is 'voice'.
'voice'+'mail'='VOICEMAIL'