Shared communication systems on which politicians often vote (5,5)

'politicians often vote' is the definition.
I can't tell whether this defines the answer.

'shared communication systems on' is the wordplay.
'shared communication' becomes 'party' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'systems' becomes 'lines' (line is a kind of system).
'on' says to put letters next to each other.
'party'+'lines'='PARTY LINES'

'which' is the link.

Can you help me to learn more?