Voorkom binnen 15 dagen dat je Simyo nummer wordt afgesloten!

Any use from the sim card or its credit would count. I suppose you apparently are not being near that sim card to sent an sms or make a call, so that leaves alternative options:

• Use Mijn Simyo to get the cheapest data bundle (costs 5 euro, but will keep it active)
• Add credit to your prepaid number… Yes, that counts too, but means you need to recharge with 10 euro.
• Let it expire and contact customer support when you return ASAP to get it back. Yes, Simyo will disconnect your number, but customer service can get it back for a short period after it gets disconnected (< 1 month). You will be disconnected for a few days if you were to go this route.
• Or possibly customer service will be able to add or remove 1 eurocent to your line to keep it active. I've seen that happen in the past, but I do not know if they can still help you that way. Customer support is not in the forum during the weekend, so you possible will hear from them on Monday.