Doro 319i ph MemoryPlus Corded Telephone

Bought this phone for my elderly mother. The amplified part of the phone worked fine but it lost its stored numbers after about 3 weeks. I contacted the suppliers and they indicated that as the phone does not have a battery it is powered by the telephone line voltage. Any change in the line voltage will cause the numbers to be lost. They advised me to try the phone on another line. I tried it at my house and again the stored numbers were lost after about 3 weeks. The phone is therefore no use to my mother because the easy dialling from the stored numbers was the reason it was purchased.