In v1.3.1 you can use the new 'international_prefixes' setting in a2billing.conf to strip any sequence of characters from the start of the dial string.
In earlier versions you might want to edit the apply_rules() function around line 1,300 in /var/lib/asterisk/agi-bin/libs_a2billing/Class.A2Billing.php and add a section like:
Code:
if (substr($phonenumber,0,1)=="+"){
$this->myprefix='';
return substr($phonenumber,1);
}
I think that should strip the + and route the call for you. You might prefer to set myprefix to 00 or 011 if that suits your ratetables better.