The VW "Premium" phone is by far the most common car phone with SIM access support. It is a separate control unit that can be combined with every radio/nav system.
Starting with the Golf 7 the "Premium" phone is integrated into the nav system and can currently only be combined with the (ridiculously expensive) Discovery Pro.
The easiest way to find out if you have a "Premium" phone is to look under the passenger seat for the control unit. It is simply contained in a foam box which is hooked to the carpet, no screws.
You can now pull it out of the foam box and check the part number.
In the recent years there have been different versions of the box with different features and problems. The following table shows all available part numbers (without guarantee for correctness):
|3C0 035 730||Nokia HT-2||DDP||No||No||No||No||No||?||*|
|3C0 035 730 A||Nokia HT-2||DDP||No||No||No||No||No||2006?|
|3C0 035 730 B||Nokia HT-2||DDP||No||No||No||No||No||2006/07?||*|
|7L6 035 730||Nokia HT-2||DDP||No||No||No||No||No||2007?||*|
|7L6 035 730 C||Nokia HT-2||DDP||No||No||No||No||No||2007/08?||*|
|5N0 035 730||Nokia HT-2||DDP/BAP||No||No||No||?||?||2007/08?|
|5N0 035 730 A||Nokia HT-2||DDP/BAP||No||No||No||Yes||No||22/2008||*|
|5N0 035 730 B||Novero HT-2||DDP/BAP||No||No||No||Yes||No||45/2008||*|
|5N0 035 730 C||Novero HT-2||DDP/BAP||No||No||No||?||No||44/2009|
|5N0 035 730 D||Novero HT-4||BAP||No||Yes||Yes||Yes||Yes||45/2010|
|3C8 035 730||Nokia HT-2||BAP||No||No||No||Yes||No||2008?||*|
|3C8 035 730 A||Novero HT-4||BAP||No||Yes||Yes||Yes||Yes||22/2010|
|3C8 035 730 B||Novero HT-4||BAP||No||Yes||Yes||Yes||Yes||22/2011||*|
|3C8 035 730 C||Novero HT-4||BAP||No||Yes||Yes||Yes||Yes||22/2012|
|3C8 035 730 D||Novero HT-5||BAP||Yes||Yes||Yes||Yes||Yes||22/2013|
|3C8 035 730 E||Novero HT-5||BAP||Yes||Yes||Yes||Yes||Yes||22/2014|
The protocol tells which display the phone module supports. "DDP" is the old red display, "BAP" is the newer white display. I personally own all devices with a * in the last column.
3G capable Premium phones have a problem with GSM-only SIM cards. This affects e.g. the Nexus 4 and some HTC phones (see also here). When you connect such a mobile to your car the phone is connected but the receiption is empty and no provider is shown.
To fix that please set the phone mode in the car to "GSM". This is what the menu structure looks like in a Golf VI:
When you have a WiFi capable Premium you can regain internet access on your phone. But Android treats a WiFi connection different to a mobile data connection. It reduces communication for mobile connections because they usually are more limited than WiFi and sometimes are far more expensive.
Android has a hidden setting, where you can specify that a WiFi access point is in fact a mobile data connection and should be treated as such.