IARC is the World Health Organisation's
International Agency for Research on Cancer. Part of IARC's
work is to classify agents according to carcinogenicity. The
classifications are not actually that straightforward when one looks
at the detail, but simplistically they are:
IARC said in 2011 that phone signals (and also, a decade earlier, power-frequency magnetic fields) are 2B, possibly carcinogenic. So what does that mean?
It's important to understand how the IARC classification is derived. Essentially they look at the weight of evidence for and against risk without explicitly considering plausibility, and that is both a strength and a weakness. As we discuss above, we expect around 20-30% of studies to show an effect even if there is no risk, because that's the false positive rate in biomedical research. With over 25,000 published studies on this subject, we certainly can't say there is "not enough information" (3), and the presence of those few thousand positive studies prevents us from saying "probably not carcinogenic" (4: in fact IARC has only ever assigned this category to one chemical, which tells you something about the process). Similarly, phone signals are not in IARC's opinion either carcinogenic (1), or probably carcinogenic (2A). The 2B rating reflects that there is some evidence of risk - but that it's outweighed by the evidence for "no risk". And that's about as far as it goes.
When expert groups and standards-setters review the science they look also at plausibility, so they consider the weight-of-evidence as IARC did and see that it doesn't suggest a probable risk, and they note that there is no plausible biophysical mechanism for harm. As a result there is absolutely no conflict between those reviews concluding that there is nothing to see, and IARC's 2B rating.
The key argument about the importance of the 2B rating is whether it has lead to limits being revised. And, it hasn't. A number of expert groups have re-iterated the current SAR limits in the intervening years, and there's not one that has advocated moving to any other basis. Moreover, we can learn from the experience of power frequency EMF. That received an IARC 2B classification in 2001, and since then the limits written by all the scientific public health bodies on the planet have remained cleaved to the known biological bases of exposure. And, we haven't really changed the way we generate, distribute or use electricity as a result.