First of all, torrent clients/peers will try to connect on TCP ports 6881 thru 6999. ISPs will apply QoS (Quality of Service) on traffic hitting these ports. QoS can be applied on traffic class (class name or CoS), protocol header description files, Real-Time Protocol (RTP) protocol port, port type, peer-to-peer traffic, etc, etc. Essentially what the ISP will do is (very cleverly) apply the lowest QoS priority to based on the traffic class for P2P traffic by matching the outbound packet with a number of rules. Essentially even if you ONLY allow your torrent client to connect on port 80, 443 or 8443 (HTTP & HTTPS ports) you will still get caught by one or more traffic classification rules. Blocking by IP for torrents is an absolutely futile effort BTW, as mentioned is some of the above posts. Blocking on port numbers alone is also completely futile as torrent clients will revert to port 80 if all else fails. Torrents clients have also started with Protocol encryption (PE), message stream encryption (MSE) or protocol header encrypt (PHE) which proved effective for a while until ISPs learnt how to identify traffic better. It's a constant cat and mouse game though to block/classify P2P traffic.
Certain types of traffic (specifically the real-time protocols) will always get the highest QoS rules applied to them, this is generally things like Skype & SIP type traffic.
