Most of the SODAQ boards have at least 1 or 2 UARTs available. These operate with 0-3.3v LVTTL levels, where 0.0V is LOW and 3.3V is HIGH.
The RS232 signal is a bit different. Given an operating voltage of 3V, LOW will be +3V and HIGH will be -3V. You will need a converter as the signal needs to be inverted and the low needs to be clamped to 0.0V.
Some Arduino boards (such as the UNO) operate with 5.0V logic levels. The SODAQ boards however, all use 3.3V.
You may cause damage if you apply a voltage higher than 3.3V or lower than 0.0V to any of the IO pins (the absolute maximum rating is VDD + 0.6 or 3.9V, and similarly 0.0V - 0.6V or -0.6V).