Mis sold PPI now with Avant Card

I made a complaint over the mis-selling of PPI to Avant Card although it happened with MBNA. I followed their procedure and I received an update/full response date. This passed and I then received a response date which has now passed. I tried ringing them but the phone is never answered and the voice mail message is in Spanish (I think!) so I'm not sure I'm leaving messages with the correct person although I think I can make out the right name. One of the letters was supposed to contain an explanation of the complaint handling process but it was not enclosed.

I guess I'm looking for advice on what to do next? I can't find any other applicable number that I could ring. Do I have to wait for them to get back to me seeing as I have left a couple of messages? Does the complaint have to be responded to within a specific time frame?