One worldwide card is not the ultimate solution.
A little hassle you'll have to accept to save a lot of money.
Your infrastructure has to be reorganised, therefore I recommend you:
Sparruf SparRuf – mit dem Handy billig ins Ausland telefonieren: cheap local number (Germany), many useful features like SMS triggering, calls directly from your computer, call forwarding, callback, free fax forwarding to your email, also mailbox forwarding to your email, possibility to call you in more than 50 countries via your user account number for national landline tariffs* (you have to pay the forwarding expenses) and much more.
Forwarding to an estonian travelsim is 19€ct/min in good quality.
The Sparruf website you can have translated with website translation on online-translator.com.
www.online-translator.com
*This option you'll see after opening an user account under "my number".
Sparruf is is mainly to receive calls, because DialNow (s.b.) is a lot cheaper.
To call others I recommend you a local SIM for data and to receive calls and
DialNow DialNow | Cheap International Calls.
Triggering is possible with access numbers, online on their website and from your handset using
http://gsm.voipbuster.com
Dialnow is a Betamax brand and does NOT work with estonian mobile numbers i.e. Travelsim or airBalticcard.
The Sparruf website you can have translated here:
www.online-translator.com (web translation).