I usually switch about once a year or so, you only save the big bucks the first time, but if you don't keep switching regularly the price will start creeping back up again.
I'm not clear why you're phoning them, you can do a search online with Moneysupermarket, Uswitch, etc. Beware that you need to untick the box that says something like "Only list products that we can switch you to via this site" otherwise you'll just get the companies that pay commission to the search engine. You also need to watch out for penalty clauses if you're looking for fixed price contracts. I usually double check the tariff on the suppliers website and then phone to switch.
It's not usually any aggro, I've only had problems once, and that was just because there had been a meter change at the same time by coincidence.