I completely agree with the sentiments in the original question: I also actively avoid calling businesses with premium numbers.
The problem is that it's typically not the sales line that is premium. In general, the premium number is on the customer support line - the number you call once you're already doing business with them; ie when it's too late to change you mind over it.
However, the reasoning behind putting a cost onto these lines is actually less customer-hostile than it appears at first glance.
In most cases, "premium" on these lines does not equate to "astronomically expensive". Premium rate lines here in the UK can potentially be up to £1.50 per minute; whereas this kind of line would normally be more like 3-5p/min.
The reason for the charge is to prevent "spam" calls. You want people to really need to call you before they pick up the phone; you don't want them calling at the first sign of trouble and having your support rep spend an hour talking them through the basics from the user manual.
The money earned from the call at these low rates is obviously never going to cover the cost of the support rep's time (not even if you spend ages on hold). But by cutting out the trash calls, they're dramatically reducing the amount of wasted time for their support staff, and thus cutting costs and giving the callers who do get through a better service.
The classic example of a free number that is abused by people making stupid calls is the emergency line (ie 911 in the US, 999 in the UK, 112 in the EU). The stats are scary of just how many emergency calls are either pranks, hoaxes or just plain stupid non-emergencies. The cost to society of all those calls is high, and the main reason for this is because it's free; people don't think about the consequences before they call because it doesn't cost them anything.
I'm not saying that your local PC repair shop's tech-team line is equivalent to the emergency services, but the same thing does happen on a smaller scale.
So don't just look and see "premium rate"; take a closer look, and consider what the rate is. If it's relatively low, the business probably isn't trying to pull one over you after all.
There are of course plenty of counter-examples, where bad businesses take advantage of captive audience and charge you excessively for calling them, but for most businesses it's not about that at all.
If you are worried about the costs, it's worth noting that in most cases there is a non-premium number you could call. In many cases it will be published as the number to use if calling from overseas. Or you could try their general office switchboard and ask to be put through, or at a push try their sales line.