Faulty Microsoft Lumia 640

I got myself a cellphone contact from cell c on the 28th August this year. when I got home the phone kept re starting itself and this would happen several times in a day I reported this at the cell c branch they took it for an OBF and I was given a new one. The very same thing happened and I returned it again and yes they gave me another new phone, the problem did not stop and I sent it again and got another one on the 23th September which did the same I took it back on the 30th September and I was told I have missed the OBF date I could not understand this but they said the only help they can give me is to take it for repairs. Till today I'm still waiting for their call to say I can come and collect the phone because when I called they said its not ready. Is there justice in any of this, as I am paying for monthly subscriptions but I do not have a phone I am not enjoying its benefits. I am not happy about the fact that they could not give me another phone and the fact that The repairs are taking too long. Now I'm thinking of taking legal actions against cell c.