With prepaid USIM, can't send SMS.

Hello, I've been using iPhone6(64G) since Dec,2014. I bought this in Korea.

So, almost all contacts in my iPhone are stored in Korean phone number system. (start with +82 country code)

But after I arrived here in Germany(country code +49), more precisely, after I changed my USIM with prepaid USIM card, I can't send SMS to my Korean contacts any more. Yes, outgoing calls to those number are working perfectly! And I can send SMS to German friends.

Here's one example how I stored the number for my Korean contact.

Korean contact Name

mobile

+82 10-xxxx-xxxx

home

+82 2-xxx-xxxx

I can make a call to above mobile phone number but can't send SMS.

Funny thing is, I've got an SMS from Facebook(country code +44) yesterday for the purpose of authentication and I could reply to that number!

Please help me. I'd like to send SMS message to my daddy in Korea everyday who only can use feature phone.

Currently, my iOS version is 9.2.1