Whirly,
I think the problem is, these fields might not NEED the phone call all the time, only some of the time... which means you can TRY just calling in by radio and that'll be fine. But it may happen, if you depend on that, that they'll say no, for whatever reason, and then you find you don't have a proper alternate. So, call in by phone if possible, but you can of course TRY just by radio and if it works, all the better!
You should notice in the AIP, that the PPR is listed for almost all "special airfields" ... at public fields usually after OT (other times... i.e. other then the regular opening hours).
Westy