Ofcom records show that numbers starting 07521 are allocated to Telefonica UK Limited. Note that due to number portability if an individual number has moved to a different operator this information is not publicly available.
Ofcom's last recorded change for this number range was May 2007.
The correct way to write telephone numbers beginning 07521 is in the format 07521 xxxxxx
For more information on tariffs and usage of numbers starting 07 please visit the page on 07 numbers