According to
this tariff info with CallYa International you can subscribe to any CallYa option ("Optionen: Zu CallYa International sind alle CallYa Optionen zubuchbar"). But "CallYa OpenEnd Internet" is NOT a tariff option, but an alternative tariff. So you can't combine CallYa International's call rates with the data rate from CallYa OpenEnd Internet.
Actually there's an option called "CallYa MobileInternet Flat" offering data for € 0.99/day, € 4.95/week or € 14.95/month even for CallYa International, which is even cheaper than "CallYa OpenEnd Internet", but except for WebSessions all CallYa data options are restricted to mere web access. So you can not use eMail or any other service except for webpages opened with your phone's browser.
If you want full internet access including eMail service, you'll have to rely to WebSessions.