SIP T-28P losing registration

[Sepp]

Hardware: T-28P, Firmware 2.70.0.140

Problem: We have 2 identical phones (same type as above, same firmware, same configuration) in 2 different places. One is hooked on a ADSL line, the other on a tv cable account. Same router (identically configured), same VoIP provider, just a different number.

The phone on the cable works fine, but the phone on the ADSL tends to lose registration after not being used for some hours. I exchanged the phones within the arrangement but still the same problem.

Does the phone on the ADSL need a different configuration (see attachment)? Any other idea?

Thanks for your help,
Sepp