Take a look into Article 725. Most garage door operators I've wired are marked as having a class 2 control circuit, so your wire would have to be one of the ones in 725.71. You can also look at Table 725.61 for a substitution matrix.

I don't have a bell transformer handy, but you'll probably find that it is marked as a class 2 power source, thus requiring one of the cables from 725.71.

Most of the 18-2 doorbell wire sold in my area is CL2, which would be a suitable substitute for any of the required cable types for a dwelling

Tom

Few things are harder to put up with than the annoyance of a good example.