More specifically, it varies by carrier legacy system. Finding the correct email-to-SMS address construction (usually some variant of phonenumber@domain.tld) may take some poking around, and it's not uncommon for the domain.tld portion to refer to some prior carrier acquired by the nominal provider (e.g.: your AT&T wireless number might have a cingular.com address, though don't quote me on that).
That said: it typically works and is fairly reliable once you've identified your gateway.
