To be able to call from abroad, your roaming has to be activated. Enter the phone number with the international dialling code of required country; the one of the Czech Republic is +420 (or 00420). If you want to call a Czech phone number, enter +420 123 456 789 into your telephone (replace123 456 789 with the dialled number). SMS and MMS should also be sent with the international dialling code +420.
If you are using the recharging payment method and you want to make a phone call from abroad, the service of direct calling only works in some networks. In the networks, which are not featured on the list, you can still receive calls, send and receive SMS, MMS and use data connection with your mobile phone.