Use a grok filter to extract the various pieces of the string into discrete fields. What's most important for your current question is that the JSON array is in a field of its own. Then have a json filter process that field, and finally you'll have to write a small piece of Ruby in a ruby filter to collect all the call id values into an array. This probably works:
ruby {
code => "event.set('call_Id', event.get('XXX').collect { |h| h['callId'] })"
}
Replace XXX with the name of the field containing the parsed JSON array.