As far as I know this kind of additional charge for dialing within the same country exists in some countries, but this should be not confused with "roaming charge" for intra-country roaming.
But if your SIM is away from the home network it is always roaming. The data will be sent back to the home country. So sending an SMS to a local number is like sending an SMS to a foreign number.
Yeah, except that somehow I found that it doesn't always work this way. I live in the UK, and when I call my mum in Poland using a British sim card, I have to pay extra for making a foreign call. But when she's over here, I can call her for free(using my domestic minutes allowance), even though I'm still dialing a +48 number, the telecom knows that the call is being routed internally and no extra charges are applied. Yes, I was surprised at that too.
