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There are several ways a compass can point askew besides cellphone proximity. Any kind of magnetic material (large chunks of iron or steel) on or about your compass or even magnetic material in surrounding rocks can give an inaccurate reading. This happened to us on Orizaba some years ago.
Learn alternative ways of finding North - learn the night sky and where the north star (or southern cross, if you are down under) is located, for instance.
That Orizaba trip, many years ago, was one where a lady in our party needed suitable boots quickly. her boyfriend, a member of REI, put a check in the mail (this was before credit cards) while REI put the boots in the mail at the same time. They arrived promptly, leading to successful ascents.
I believe his membership number was about 9000.....