FXS doesn't do polarity reversal on answering calls

I am a extreme newbie in setting up Asterisk. Currently I am trying to setup the versions of Asterisk and dahdi below:

DAHDI Version: Echo Canceller: MG2

I installed two AEX410P cards with four FXO modules on card1 and two FXO two FXS modules on card 2, the FXS modules are connected with Analog DDI lines from local ISP in Hong Kong to take incoming calls. They are configured to use loop start signalling for the moment.

Problem: when I used my mobile phone to dial in the DDI lines(FXS ports) from outside, Asterisk could establish the conversation with internal SIP clients but the call dropped within 2 minutes. I spoke my problem to the ISP and they said it might be my Asterisk unable to provide a ‘polarity reversal’ on the line voltage to give an indication to the calling party that the call should go on. Everytime I try the dialing with my mobile, the mobile keeps showing it’s dialing but the call timer never starts to count, even if the sip client had picked up the phones and started conversations. I would like to know how could I create this ‘polarity reversal signal’ in Asterisk so that the call will not drop, or should I need to fix it in other approach.

I am so desperate to get it work now, anybody here please help.