warranty nullified unduly

the first Sony Ericsson handset which i got through a contract only lasted me 8 months and it was replaced with a new handset as the problem with the charging system was beyond repair - the second device only lasted 4 month also giving me the same problem - the second device (just like the first one) came with a full 12 months warranty and now Cell C is refusing to honor the warranty on the second device even though both devices has the same problem. i believe both devices were manufactured to last the full term of the warranty and in case of a discrepancy it would be covered by the manufacturer, therefor cell c should liaise with the manufacturer to compensate the shortfall rather than alter my contract conditions.