After concerted efforts by the EU to reduce roaming charges between member states, it is forbidden as of 15/06/2017 to apply roaming charges for calls, SMSs and data service within the EU. Therefore, when travelling from one member-state to another, you will be billed just as in your country of residence. Especially for outgoing calls, the method of billing of calls to other mobile/landline operators in your country of origin shall be taken into account. Apart from the above way of billing, mobile operators may offer some alternative programs for international roaming, with different terms of use and billing.
In non-EU countries, charges for international roaming are not governed by any regulation, so the amount is determined by the commercial policy of each provider.