If you are traveling to Belgium you can continue using your iPhone (or iPad 3G or Android phone) without spending a lot of money on roaming costs.
When your smartphone is not sim-locked:

Go to a
BASE store (you can find them in the biggest shopping streets across the country) and buy a prepaid card. The SIM card will cost you €15 but you can use that amount to call or to activate the
mobile surf option. The disadvantage is that you will get a Belgian mobile number. Note: when using with the iPad, you will need to cut the SIM card as till today, in Belgium, we don't use micro SIM cards.
BASE is only having 3G in the major cities. Elsewhere they have EDGE.
When your smartphone is sim-locked:

Bad luck for you although there is a (not that cheap) workaround: Buy, before you come to Belgium, a
Mifi that is not sim-locked. Go to a
Discovery store (that is a Proximus/Belgacom store) and buy a
reload card. Put this card in your Mifi, create a WIFI network and surf over the WIFI network with your smartphone. The daily cost is €2,5. The advantage is that you stay reachable on your known mobile number.
When you want to surf the net with your laptop:
Buy a 3G USB stick in Belgium (in Belgium we don't know SIM-locked phones or 3G sticks) in a
Discovery Store and buy a
reload card. Install the software that comes with your 3G USB stick on your laptop, insert the SIM-card and you are ready to surf the net.
And what about Mobistar ?
Mobistar is the official iPhone partner in Belgium. Their network is OK although today Base and Proximus are cheaper for prepaid cards.
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