I've been to Germany today and while I was there I bought some credit for O2. I tried to top up from Germany, but when I chose top up with a voucher from a star alliance member (or something like that) I was told I chose the wrong code (select 1/2/2 in the menu as I remember well). Back in the Netherlands I tried again but there the complete option was gone. With another phone I called customer service and the lady on the other side was so friendly top up my german O2 cards for me.
Quite a bit of work for a top up

Active sims NL Telfort, KPN, Vodafone, TrendCall, T-Mobile
DE Simyo
CH M-Budget
PE Claro
INT Air Baltic Card, GeoSim
Non-active sims NL Qick, Easymobile, Tele2, Simyo, Rabomobiel, Vectone, UPC, GT Mobile, Orange, Hema, Lycamobile, Carey
TZ Celtel
Malawi Celtel
Zambia Celtel
SA MTN, Vodacom
INT UM, UM+, Go Global Sim, Max Roam, 09 Sim