Phenolic is very tough stuff but it can be worked with carbide tooling. If the table is not already mounted, I'd be tempted to cut the slot with a dado head in the table saw. Otherwise, you can use a router and straight-edge guide. just take small bites, increasing the depth of cut in small increments.
Kind of like Jerry says, the dado stack will make things go faster. I would still clean up with a router because, in my experience, dado stacks rarely leave a smooth flat bottom, but if you get it close, then you can easily clean up the last bit with a router.
That phenolic will dull the crap out of your dado blades, even just cutting one dado for a miter slot. I would definately use a 1/2 shank carbide tipped straight cut router bit and take small cuts until you reach the desired width and depth. Better off dulling a router bit as opposed to dulling a set of dado's. Plus it will come out flatter just take your time.