SpiritualISM IS a religion. Its meeting places are called churches, and are chartered as such in the USA and in the UK.
To some people who consider themselves to be SpiritualISTS, it is a / their religion. Other Spiritualists regard it as a practice, and regard themselves as members of some other, or of no particular, religion.
Spiritualists don't 'just' talk to 'dead' people. The essence of Spiritualism -- and what makes it different from other religions -- is that Spiritualism offers EVIDENCE, not merely faith and hope, that life continues after the death of the physical body. Spiritualism has shown us that there is no death, other than the death of the physical body.
Spiritualism also offers revelation from spirit sources. As does Christianity, and other religions.
Someone asked at the outset why it's not listed in telephone directories under religion. My response would be that telephone directories are hard to find any more!
But more to the point: if there's a Spiritualist church in your area, and there's a telephone directory for your area, it quite possibly IS listed therein, under Religion or Churches or Houses of Worship, or whatever heading the directory uses.
If you live in the UK, there probably is a Spiritualist Church in your area. If you live in the US, there probably isn't.