Telit FAQs, Part 14: For xE910 modules, what are the minimum RS232 (UART) connections that I need to have?

The absolute minimum with no flow control is RXD and TXD.

For hardware flow control, the minimum lines are TXD, RXD, CTS and RTS

Note:
If you want to have flexibility, adding DTR and RING are helpful. DTR can be used to toggle the module from data online mode to command mode as well as control the power save mode of the module. RING can be used to know when incoming events occur without having to monitor explicitly for URCs, though monitoring URCs and also RING is the best combination.