Hi everyone,
The U201's UART pins are not exposed via the headers.
However, there's an example in the library of how to bridge the USB CDC port and the U201's UART. No need to deal with the nitty gritty details of the Sercom, everything can be accessed via SerialGSM.
SerialGSM will also take case of the flow control (CTS and RTS) pins connected. I'm not sure if it will work with U-Center, but please let us know if it works or not.
Out of curiosity, what features are you looking into? We'd be open to integrate them into the MKRGSM lib ...