MOD-GSM doesn't transmit properly data on the UEXT of another Olimex board. Where is the problem?
You might need to cut the UEXT cable connecting the MOD-GSM with the board so only data call signals are left, refer to MOD-GSM with DUINOMITE boards (CTS is held at high position, which means it will not send data). Keep in mind that some other boards can also interfere with MOD-GSM's signals. If the host board has any signal on the I2C and/or the SPI on the UEXT. There is very high chance of erroneous behavior.