I think the big success of dual-band phones, at least here in Italy, was with Wind and the
Siemens C25, a cheap and easy phone, in 1999.
Wind had only DCS 1800 coverage, but had roaming agreements with TIM out of the 8 main cities where they had their own coverage. Then national roaming with Omnitel (then Vodafone) arrived too... so a dual-band phone was needed unless you lived in one of those 8 cities (such as me!).