There must have been time, when you most likely would've received some major antidotes for talking about operating systems that could fit into a mobile phone. Radio phones used analog technology, and they were like tuned musical instruments, only that these devices operated on a specified band of electromagnetic spectrum.
The early mobile phones were set up at the factory, and then used in unmodified form for their lifetime. The things you could change were ringing tone, and your address book contents.
The phones certainly had no "disk drives", no code pointer, stack pointer, internal memory, or the kind. At least this is how it looked to the surface - for the end user. The phones indeed had already quite sophisticated operating systems, but they were closed-source and hidden from prying eyes, running stealth within the phone. Just like nowadays you can't modify the operating system of a washing machine... Because there's simply no compelling reason.
The OS's role was to handle things as complicated as multitasking; the early WAP browsers, which were supposed to bring mobile Internet to European countries, probably contained tens of thousands of lines of code.
It would be most interesting to know more about the early history of mobile operating systems. I had a British Psion Revo pocket computer, which had EPOC operating system on it. These little computers were called organizers, or PDA (personal digital assistant). In 1986 Psion released the Organizer II, which marked a milestone in the march of PDAs.
Later on, Palm Computing and many others created a variety of these devices. They almost invariably lacked proper data connectivity, which perhaps made their era shorter.
Without slick network access, you had to use all kinds of docking solutions to wind up the PDA with a PC - and your data would not be synchronized in real time, nor was sending or receiving possible when the user was away from those "PC access points". It wasn't very beautiful at all. We were still talking about sales figures like 1 million units per year.
Later on, this EPOC would be the core on top of which Symbian was built. Nokia, in 2001, had produced its flagship communicator 9210 which ran Symbian OS (which was a rename from previous EPOC32). Symbian grew, measured by units shipped. In 2007, there were 126 million units sold by end of March, 2007. Nokia was also active on "non-smart" phones, ie. ordinary mobile phones. It lost focus in the smartphone sector, partially because of the steep learning curve of new programming developers trying to enter Symbian world. In addition, Nokia kept the development environment semi-closed, requiring participants to register for a fee.
Now, years later, in 2011 as Microsoft and Nokia have decided to use Microsoft's Windows Mobile Phone (WMP) platform as Nokia's future smartphone operating system, the Symbian was given to a consulting company called Accenture, along with some 3000 developers from Nokia. One good question which has been circulating in media is: What happens to Symbian? And the developers? Even though the sentiment is generally a bit gloomy, we might have surprises along the way. As Finns say; "Älä heitä kirvestä jorpakkoon, vaan tee ilmaveivi."