First off, your R15 should scroll off the oldest call once 25 are reached. Both of mine do that. That's the first time I've heard about the unit freezing on the 26th call like that. The other problem, however, isn't new. I had that originally on my 500. It worked like that (only allowing 1 or 2 calls in the log and not showing any new calls until the log was cleared out, and even then it was iffy - it wouldn't always take new calls, even if the log WAS cleared out). A month or two ago, I did a reset for a different reason, and my R15 went through the full setup mode all over again (not sure why, but apparently that's a known problem as well - but doesn't happen with every reset), and after that my caller id worked fine. And has ever since.
I'm guessing that you have a model 500, based on the issue that you're mentioning. The 300s seem to have a different problem with caller ID (receiving "phantom" calls that aren't actually made - and all of which have the same date/time of Christmas Eve at 1:24 am). And I've yet to hear of anyone having issues with caller id on the model 100...