__callStatic is not called?

You can not staticfy your object methods through __callStatic calls in PHP. It will only be invoked when the method so far does not exist (incl. not being visible from the calling context). In your case Test::getSomeVar() is already defined.

Because of backwards compatibility, PHP does not check if only a static method exists, but actually if generally a method exists.

In your case you are calling the non-static method statically, so $this is not defined because __callStatic has not been invoked. If you would have enabled warnings and notices to the highest level (recommended for development), PHP would have warned you about that.

The correct usage therefore is:

echo Test::instance()->getSomeVar();

As with any other implementation of a Singleton.

So you are just using __callStatic wrong, it only works for methods not yet defined. Choose another tool/design for the job, like the aggregation example you use with your Blah class.

A further note:

You should normally prevent to use any static context in PHP anyway, especially as you exploit a lot of magic functionality here, too, which is another smell. It looks like you have a pile of design issues before you even finished your code. You will only increase the likelihood to run into hard to debug and maintain code with all this. But that just so you do not say in some time you have not been warned.

See Who needs singletons? in case you want to learn more.