Is there a possibility to retrieve the "actual" Mobile Country Code of the iPhone?
If I use the conventional method over the CTCarrier I only receive the MCC of the SIM-Card, but I want the Mobile Country Code of the Roaming Provider.
No this is not possible in the current iOS SDK.
I don't believe this is possible with a private API, but you can use a third party service such as http://ipinfo.io, which I built. Here's the standard API response, which will include carrier details in the org field:
$ curl ipinfo.io/24.32.148.1
{
"ip": "24.32.148.1",
"hostname": "doc-24-32-148-1.pecos.tx.cebridge.net",
"city": "Pecos",
"region": "Texas",
"country": "US",
"loc": "31.3086,-103.5892",
"org": "AS7018 AT&T Services, Inc.",
"postal": "79772"
}
Custom packages are available that also include the full mnc/mcc details for mobile IPs though. See http://ipinfo.io/developers for details.