as long as yuse MGCP
you have to do it throut translation pattren in CUCM
lets say your internall numbers are in the range of 4XXX
now the call come from PSTN
make the gateway CSS has access only to translation pattren you will create
make the pattren same as your internal range
4XXX
give the translation pattren CSS has access to internall numbers within the range 4XXX
in the calling party trnsformation section
Check the Use Calling Party's External Phone Number Mask checkbox
and in the calling mask
put the number of digits you expect from PSTN with prefix 9
for exale 9XXXXXX
leave the called party transformation mask balnk to keed the number the same
i mean called number
and bellow link will help you
http://www.cisco.com/en/US/products/sw/voicesw/ps556/products_tech_note09186a00805b6f33.shtml
good luk
hope this helps