When was the change made from a jury alone to a telephone voting system in the Eurovision Song Contest?

Explanation

After a trial in selected countries in 1997 (Switzerland, the UK, Sweden, Germany and Austria), the combination of telephone voting and jury selection was introduced in 1998. The current system whereby each country awards 12, 10, 8, 7, 6, 5, 4, 3, 2 and 1 points respectively was introduced in 1975. 12 points is the highest you can be awarded.