I can tell pretty much immediately if I'm about to receive an outsourced telemarketing call... my phone displays caller ID as "UNAVAILABLE" which usually means it's an out-of-country call.
Then there's the inevitable propagation delays caused by them using cheap satellite routes which makes an already difficult conversation (trying to understand a thick Indian accent) even more entertaining
Most of them claim to be calling from Telstra which is the largest Australian telco (and I believe the largest Australian company overall) but they're really calling on behalf of reseller telcos who want me to switch billing to their company. Pretending to be from another company has gotta be halfway illegal.