There is no new area code.
There is no 02920 area code, and never has been.
In April 2000, Cardiff telephone numbers changed to the 029 area code and all Cardiff six-digit local numbers were changed to eight digit local numbers by prefixing the local number with "20".
Within Cardiff, omit the 029 area code to call any other Cardiff eight-digit number.
Cardiff residents continued to be issued with those eight-digit 20xx xxxx numbers for the next decade.
Now that all of those 20xx xxxx numbers have run out, Cardiff will use eight-digit 21xx xxxx local numbers from now on.
There's no 'change'.
There's no new area code.
The whole point of the Big Number Change in April 2000 was to create more capacity for local numbers in various cities. A decade later that extra capacity is now starting to be used.
Stop referring to these as 02920 and 02921 numbers. There is no such thing.
They are 20xx xxxx and 21xx xxxx numbers in the single 029 Cardiff area code.
Correct presentation is (029) 2xxx xxxx for domestic use and +44 29 2xxx xxxx for international use.