Best German online phonebook should be
DasÖrtliche Online, but a phonebook won't help you ...except when you find someone from the same street to call and ask. Said this, I would suggest Wittgensteiner Straße instead of Wittgensaunder Straße. The phone numbers of #21 are 02733/2408 (Herbert Jentsch) and 02733/128035 (Reinhard Jentsch). Good luck!