Depressing high toll overwhelms hospital department (7)

'depressing' is the definition.
The answer and definition can be both to do with contact as well as being present participle verbs.
Perhaps you can see a link between them that I can't see?

'high toll overwhelms hospital department' is the wordplay.
'high toll' becomes 'ding' (I can't justify this - if you can you should give a lot more credence to this answer).
'overwhelms' means one lot of letters goes inside another.
'hospital department' becomes 'ent'.
'ding' going around 'ent' is 'DENTING'.

Can you help me to learn more?