From what I can tell, Apple AirPods use Bluetooth Smart, part of Bluetooth 4.0 BLE, to pair with devices. Bluetooth Smart supports pairing without a passcode.
AirPod Compatibility shows that it requires an iPhone 5 and later, iPad mini 2 and later, or iPad Air and later. All these devices have Bluetooth 4.0. Granted some other devices such as the iPhone 4S have Bluetooth 4.0 but are not listed as compatible.
http://www.apple.com/shop/product/MMEF2AM/A/airpods
I tried looking up details about your handset, but there wasn't much regarding what version of bluetooth is supported:
http://www.gigaset.com/hq_en/gigaset-s850h/
If I had to guess, I'd say the Siemens device does not meet the Bluetooth 4.0 requirement. Just to be clear, I could be correct, only half way correct, or completely wrong.
Bluetooth Smart info:
https://serialio.com/news/what-ble-bluetooth-smart
http://www.laptopmag.com/articles/just-what-is-bluetooth-4-0-anyway