Redirect running call

Hi, from time to time people call me who actually want to talk a colleague of me. Therefore it would be useful to redirect the running call to someone else. I have contacted my provider (1&1 Germany) but they don't have such a service. Is there a way to do that on android? I use CyanogenMod 12.1 on my Nexus 5.
PS: I've tried to open a conference an leave it after my colleague picked up but that kills the connection between every caller.