State of knowledge
Softphone calls made on a smartphone cannot be forwarded.
For iOS: If the smartphone was in locked mode before accepting the call, you must first jump to the app via the "ProCall Mobile" button and can then forward the call directly via the "arrow" symbol.
For Android: Calls can only be forwarded if the call account is deactivated for ProCall Mobile (under Phone → Settings → Other call settings → Call accounts → All call accounts). When using the ProCall call account the forwarding of calls function is not supported by the operating system. If the ProCall call account is deactivated, the call can be forwarded by clicking on the "arrow" symbol.