Well, maybe I am missing something but I do not see a 12.4(15)XW release on CCO for the 2851 platform. I see references to it in documentation but I don't see actual release notes or a download link. So, where exactly did you see this particular version for this platform?
Based on the PDF provided by the OP I think he is trying to add a fax module. In the PDF it is stated a few times that you can use 12.4(15)XW or 12.4(20)T. What this basically means is that these are minimum IOS version levels that you need to use that module. You would not want to run the minimum in most cases because software defects are abundant and seem to be endless. IOS release trains are somewhat over complicated in my opinion but here is my quick take. The XW is a early deployment release. These releases are "one offs". Usually added to incorporate a new piece of hardware. Sometime after these releases come out (again, to get the product to market) the code is incorporated in a T-train (in this case, 12.4(20)T). The idea is that the T-train code is used to introduce features/hardware/etc. that will be rolled into the next main-line release (in our world that is 15.0).
Back to your issue. I can't find 12.4(15)XW but apparently p.bevilacqua has been able to find it. Maybe he would be willing to provide the link. If that doesn't pan out, I suggest that you look at releases after 12.4(20)T (including the latest service release for 12.4(20)T). They go up to 12.4(24)T. T-trains are touchy so you have to do your research and testing. I have had decent success with 12.4(20)T until recently (MGCP and T.38 fax, no joy). I have also used 12.4(24)T with OK success thus far. I know that p. bevilacqua doesn't like the 12.4T train "because its buggy". My opinion is all of this software is buggy. You have to identify the minimum release that will work with your hardware. Go to the latest minor/service release and research the bug toolkit. Oh, and always test.
OK. Down the rabbit hole I go.
HTH.
Regards,
Bill
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