From about 2002/2003, yes, but by then the operators were messing the manufacturers around. Nokia's golden era was approximately 96-2002 before this and the S60 mess really bit.

Vodafone went as far as commissioning handsets directly from co-operative manufacturers and this changed the game completely as the message was heard that unless you play their game you aren't getting sold.

Only the deal Apple got from AT&T remotely affected this status quo. Android was originally designed for precisely the kind of operator customisation we're complaining about, as is Firefox OS, but it's only thanks to Apple (and I loathe the way iOS has gone) that we have any choice about this stuff at all. The fact it took them to stir things up is justifiably embarrassing to the telecoms industry.