The occasional surprise is pleasant. Regular ones, however, start to fray the nerves. Yesterday Nokia results were good, and Motorola's poor, but the fact that share prices for both moved by a tenth should alert investors to the challenges of mobile phone making.

At Motorola the question remains when the ebbing tide will start to turn. Missed estimates and a realisation that the smartphone market is proving harder to crack than anticipated sent shares plunging yesterday. Given that the company's turnround strategy rests on its new smartphones, and that overall volumes continue to tumble – fourth-quarter unit sales of 12m compare to 19m a year ago – profits remain distant.

Meanwhile, Nokia bounced as the Finnish phonemaker, the world's largest, took back share of the smartphone market in the course of selling 126m phones in the fourth quarter. But yet another results day with a dramatic share price move makes it clear that investors cannot predict the company's fortunes. It is hard to have any confidence that Nokia will hang on to its 40 per cent share of smartphone sales (up from 35 per cent in the third quarter).