I always thought Symbian was made by RIM. I could be wrong though.... Every other phone uses WM and then there is the way behind the times Palm OS only found on various flavors of the Treo.
Just to clarify [from wikipedia]:
"Symbian OS is a proprietary operating system, designed for mobile devices, with associated libraries, user interface frameworks and reference implementations of common tools, produced by Symbian Ltd. It is a descendant of Psion's EPOC and runs exclusively on ARM processors.
Symbian is currently owned by Nokia (47.9%), Ericsson (15.6%), Sony Ericsson (13.1%), Panasonic (10.5%), Siemens AG (8.4%) and Samsung (4.5%). Although BenQ acquired the mobile phone subsidiary of Siemens AG, the Siemens AG stake in Symbian did not pass to BenQ."