UART1 not responding



I am using Q2686 modem in a custom board. The problem is that the modem doesn’t answer to an AT command or any other command. Signal gets into the modem trough an RS232 interface, TxD signal is high, asserted by the modem, but never transmits! Boot signal is OFF, ON/OFF signal is ON. The modem is supplied at VBat at 3.3V. Regulated 2.8v and 1.8v are OK.

Any suggestion about what could be generating the problem?