Gigaset DA710 is suitable for Proxpl systems

I want to replace with the DA710 a defective phone to my analog Proxpl plant AMEX 802. Outgoing calls were performed. For incoming calls, the connection was lost after the first ring. To control I tested the DA710 to a Auerswald Compact 4410 USB system. Again, the same behavior. After disconnecting the DA710 incoming calls could be carried out as usual at both plants. After email and telephone contacts with the manufacturer Gigaset me an undocumented coding (after reset to its factory defaults: Menu 1 25 0) called; these yielded no success. The supplier lets-sell! from Amazon MarketPlace currently refuses to take the unit to full reimbursement back even though it does not comply with the tender specifications.

Additional: After several mail contact and additional complaint at Amazon lets-sell! after more than 2 months refund the purchase price.