Depends on their rate. Keen does pay for the 3 minutes, but not at the adviser's rate. So if their rate is higher, they may feel it's not worth it for them to take a 3 min call. Can't get much of reading in either, in 3 mins.
But it's their risk, really, since that new caller could potentially turn into a long term client (income stream). I assume that readers that don't take new callers have gotten enough of the 3 mins and done calls to not want to deal with it anymore.