I love trying get to the PPI callers to explain to me what bank accounts, insurance, credit cards are, finally saying that I might have had a bank account, or credit card, then try to get them to explain to me what the PPI is. I had a guy on the phone for about 40 minutes one day while I was waiting for the ferry.
The best thing you can do is get connected to a human as quickly as possible, so you can soak the call.
The other day I didn't really have time to take one of these calls, but I pressed 3 to be connected to someone, and left the phone. After about 5 minutes of being on hold, leaving the phone off the hook, I then heard someone going hello for about 2 minutes.
Another tactic I have is to claim some interest, but that I have to call them back as I am changing phone numbers/jobs/houses, or that I don't have time in the next few weeks, but when I find a moment I will call them back. I refuse to give a 'good time to call' because I am so busy, but when I have a moment I will return the call. This is an amusing tactic because most of the they claim they can only make outgoing calls, and that I will have to give them a time. I generally suggest that them must have an incoming phone number for other purposes, so surely they could give me that number. Usually they decline, so then finally I try to get the persons phone number on the other end of the call. They generally say that is their private number number, to which I say that this my private number, so it really would be quite fair for me to contact them on their 'personal' number.
This seriously works, as I only get one call every month or two. I have also gotten a bit of a reputation at work for these types of calls and my co-workers have not end of amusement that I engage with these callers in this way.