I guess you are referring to their "internet support" number, 0905 566 0000 at £1.00/min. They also have a call centre with more limited opening hours, 0871 246 0000, 10p/min. If you call the latter, do you have to pay extra for the flight as it's no longer an internet-booked flight? In that case, you have to weigh up which is cheaper. The premium rate number is paying for the service, which would inevitably be more expensive on a different number.
From the prices for the number in each country, the UK 0871 number is actually a good deal! The 0905 number on the other hand seems among the most expensive.
http://www.ryanair.com/site/EN/faqs.php?sect=CONTACT&div=call_ctrUnfortunately, the answer is ultimately that budget airlines can offer cheap prices because they charge an arm and a leg for any extras, including making changes for bookings. People who have to make changes subsidise those who don't.