Upgrade but 2 months later, still no phone

I upgraded on 6 June 2015 to a Sony Experia Z1 Compact. After approximately 3 weeks the phone suddenly just stopped working and I took the phone back on 26 June. They advised that the phone had to go back to the technical department and that due to the strike it would take longer, instead of the normal 2-4 working days, it would take 10 - 14 working days. After 19 working days I tried to phone the branch, but to no avail, as the number just rang and nobody picked up the phone. I then went into the branch out of desperation and was told that the phone has not yet been received back. I demanded feedback which I got the next day, advising that they will replace the phone, but that there is no stock in the store. They would try to find a phone for me. After another week, hearing nothing from them, I went back to the store and was told that I could get another phone (different brand) but that I would have to pay in the difference. They promised to phone me today to advise what the difference would be. Needless to say, nobody phoned me and I also once again couldn't get through to their phone lines. I am desparate and will take further steps if I don't have a new phone by Tue 4 August.