I called CPW today and they were great. Took it to my local outlet and they swapped it out for a new handset without any hassle at all. They have a 28 day exchange policy which they totally seem to honour.
Not had a lot of chance to test the replacement, but the calls I have made have been crystal clear with no noise, crackling, or distortion. Will update again after a few days.
To re-cap: I had crackling and distortion on the majority of calls but only when using the earpiece - sometime it was worse than other times. It was always fine when in speaker mode or using bluetooth. New handset doesn't seem to have this problem.