Telephone It is advisable to buy an Indonesian SIM, these are cheap to buy and you can easily send SMS to the Netherlands. You can top up your calling credit (pulsa) anywhere in Indonesia. XL or Telkomsel are good providers.
TIP!!! Do not pay attention to price differences of SIM cards from the same provider. Indonesians are willing to pay more money for the telephone number. The price difference is in the number not in the quality of the provider.
Post Post to Europe takes one to two weeks. It is best to post cards and letters at the post office and ask if they want to stamp the stamps directly.
Internet It is a bit dependent on the island where you are, but the internet is becoming more normal in Indonesia. Many hotels offer it (also wireless / hotspot). However, do not expect fast connections. Internet is still not standard on the smaller islands.
Tip!!! Using your phone with an Indonesian SIM is very cheap and easy. They often cut the SIM card to the correct format on your phone!
Voltage 220 volts is common in Indonesia, especially in hotels and restaurants. A plug as known in most Western countries is also suitable for normal use. For the exceptions, a global plug (ANWB or duty-free at Schiphol) is of course always handy and does not take up so much space.