You can use REGEXP_REPLACE since Oracle 10:
SELECT REGEXP_REPLACE('+34 (947) 123 456 ext. 2013', '[^0-9]+', '')
FROM DUAL
This example returns 349471234562013.
Alternative syntaxes include:
-
POSIX character classes:
'[^[:digit:]]+' -
Perl-influenced extensions (since Oracle 11):
'\D+'