I currently have the Vodafone Callya plan and have quite a bit off Guthaben leftover inside, if I want to switch to Telekom Magentamobil Prepaid L plan, how would I do that? Do I have to contact Vodafone myself for the guthaben to be taken out or will it be transferred to Telekom? 

It's quite hard to get in contact with Vodafone lines and my German isn't good enough so if Telekom would be able to directly do this it would be so much easier for me.