def ack_answer(opts = {})
      msg = <<-BODY

ACK [next_url] SIP/2.0
Via: SIP/2.0/[transport] #{@adv_ip}:[local_port];branch=[branch]
From: "#{@from_user}" <sip:#{@from_user}@#{@adv_ip}:[local_port]>;tag=[call_number]
To: <sip:#{to_addr}>[peer_tag_param]
Call-ID: [call_id]
CSeq: [cseq] ACK
Contact: <sip:[$local_addr];transport=[transport]>
Max-Forwards: 100
User-Agent: #{USER_AGENT}
Content-Length: 0
[routes]
      BODY
      send msg, opts
      start_media
    end
def ack_answer(opts = {})
      msg = <<-BODY

ACK [next_url] SIP/2.0
Via: SIP/2.0/[transport] #{@adv_ip}:[local_port];branch=[branch]
From: "#{@from_user}" <sip:#{@from_user}@#{@adv_ip}:[local_port]>;tag=[call_number]
To: <sip:#{to_addr}>[peer_tag_param]
Call-ID: [call_id]
CSeq: [cseq] ACK
Contact: <sip:[$local_addr];transport=[transport]>
Max-Forwards: 100
User-Agent: #{USER_AGENT}
Content-Length: 0
[routes]
      BODY
      send msg, opts
      start_media
    end
Acknowledge a received answer message and start media playback

 @param [Hash] opts A set of options to modify the message parameters