Keeps ringing up causing delays

I believe the answer is:

holdups

'delays' is the definition.
('holdup' can be a synonym of 'delay')

'keeps ringing up' is the wordplay.
'keeps' becomes 'holds' (holding is a kind of keeping).
'ringing' indicates putting letters inside.
'holds' enclosing 'up' is 'HOLDUPS'.

'causing' acts as a link.

I've seen this clue in The Telegraph.

Want a hint initially instead of a full solution? Install my app