[how to hack OTP site]

HELLO ...
I want to hack or bypass or find OTP(one-time password) that this site sends to mobile phone numbers.

here is inspector of the site when you want to input activation code:
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<form id="fm" name="fm" method="post" action="/user/signUp.xhtml" enctype="application/x-www-form-urlencoded">
<input type="hidden" name="fm" value="fm">
<div id="fm:step2">

                <div class="form-check">
                    <div class="mobile-number-text"><input type="text" name="fm:j_idt15" autocomplete="off" class="text-field" disabled="disabled" placeholder="mymobilephonenumber">
                        <img src="assets/images/phone.png" class="mobile-number">
                        <label class="form-label">plz input activation code:</label><input type="password" name="fm:j_idt17" autocomplete="off" value="" dir="ltr" maxlength="6" tabindex="4" class="text-field mobile-pass allow-numbers-only" placeholder="activation code">                        
                    </div>
                    <div class="form">
                    </div>
                </div>

                <div class="clearfix">
                    <div class="col-xs-12 col-sm-4 col-sm-offset-4 col-md-6 col-md-offset-3 col-lg-6 col-lg-offset-3"><input id="fm:j_idt25" type="submit" name="fm:j_idt25" value="check activation code" tabindex="6" class="btn btn-secondary-dark btn-block" onclick="mojarra.ab(this,event,'action','fm:step2','@form',{'onevent':function(event) { handleMessage(event, 'fixed');$('.allow-numbers-only').numbersOnly();$('#fm\\:step2Captcha').val('');if (event.status == 'success') { refreshCaptcha('fm:step2CaptchaImage');}}});return false">
                    </div>
                </div></div><ul id="fm:message"><li class="tc-red">     </li></ul>
<input type="hidden" name="javax.faces.ViewState" value="-5386286413554590238:-6133632685432798175"></form>

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this value="-5386286413554590238:-6133632685432798175"> is Variable for each activation code, for example activation code for this value is: "174944"

what kind of algorithm is this value? is there any way to decode these type of algorithm?
is there another way to hack?

 

plz help ...