Many/most hospitalists don't take "call" per se, since a lot of them work 12 hour shifts. You can't really dictate your hours in most hospitalist job, but there is some wiggle room in a lot of them (i.e. can take more shifts if you want to make more money). I have seen part time hospitalists, but usually they worked out this deal after already having been full time for a few years.
For subspecialists, you generally can't dictate that you won't take call, if you are working for a hospital or group practice. If you go into practice for yourself in something like GI, at least in theory you could refuse to take all hospital call, perhaps, but part of it depends on what are the rules for hospital privileges in the hospital(s) where you want to admit patients. Many hospitals require all docs on staff to take some call - it's not something anybody likes to do, but somebody has to see the consults that come in the ER at night...
I have heard of GI docs who don't take call, however...probably if you don't ever want to admit any patients to a hospital, you could fashion yourself an outpatient-only GI practice, assuming you have some sort of surgicenter or office where you can do stuff like colonoscopies.
If you don't want to take much of any call, then I'd do something like rheum, endocrine or allergy/immuno - for these you'd have very little call, esp. for allergy/immuno.
