This seems like a perfect example for the new crunch..
First cd to crunch directory ;
Then based on a 10 digit number pre-fixed with '512'
or using the charset.lst file to set characters as numeric only;
./crunch 10 10 -t 512%%%%%%% -o tel.txt
./crunch 10 10 -f charset.lst numeric -t 512@@@@@@@ -o tel.txt
I would try it without the write to file option first to make sure it does what you want ;
Below will simply print to screen.
./crunch 10 10 -t 512%%%%%%%