If your trunk requires 0xxxxxxxxxx then the added outbound route entry should be
prepend 0 prefix +44 match pattern xxxxxxxxxx
This only works for UK calls. You can add another route for other countries.
If the trunk accepts 0044xxxxxxxxxx then the added outbound route entry should be
prepend 00 prefix + match pattern xxxxxxx.
This should work for any country.
If the trunk accepts +44xxxxxxxxxx then the added outbound route entry should be
prepend (blank) prefix (blank) mach pattern +xxxxxxx.
This should work for any country.
If you still have trouble, paste the Asterisk log for a failing call at pastebin.freepbx.org and post the link here.