jar files usually contain Java applications. If they are designed for phones (using Java subsets such as J2ME, MIDP) then they will work on the phones that supports them.
Symbian OS has nothing to so with them, except that the Symbian powered phones also support Java thus those Java apps are likely to work on Symbian phones too.
There are other phones, much more cheaper and therefore much more widely available, which do not support Symbian OS but do support Java games/applications. An example is the Nokia phones with Series 40 UI platform.
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