That can mean anything really, like retaining the time, or date for example

Each Telematics system is different - it will depend on the vehicle, its electronic architecture, how the data is handled (by the BSI or indirectly by the BSI and the Head Unit and / or the Multifunction Display Screen for example.
Some vehicles are VAN, some are CAN VAN some CAN CAN and now there are newer and faster data protocols, to cope with the additional safety features being added that need fast reliable communication with the vehicle's other systems. A lot of these are centrally controlled from a touch screen like the newer ConnectNav systems.
I think you just have to accept that your car is about 12 years old and there are limitations to what you can do with the technology.
If you really want to have a different option, then consider getting something like this, which will allow MP3 to be played from a mobile or a USB or a Flash Card and transmit to the Head Unit via FM - a simple cheap solution and at £11 it won't make you bankrupt

This will allow you to keep all the functions of your RT4 without cannibalising your vehicle for the next owner.