I Can not place a call in battery save mode. Why is this? I get call-error/could not connect.

Turning on power save mode makes it impossible to place a ordinary phonecall. Turning the PSM off again, makes it possible. Not really my expectations on the power saving... can I customize the behavior of the power save function or is this a bug? Running an iPhone 6 with ios10.2 and is tested on another phone, with iOS 10.0 with same problem.