Is there a way to forward online calls to cell network? I am traveling abroad and roaming costs are extremely high. Therefore, I thought about the following. I would leave a smart phone at home, connected to my home wifi. Then I would call that phone via Skype, Hangouts, whatever online service and the phone would forward my call through the local mobile network. This way I could make local calls via web in my home country.

Is there a way to do this?
Unless you have a particular reason for wanting to route through a secondary device, a more direct approach would be to investigate apps that are available to you that use wifi-calling. These can be used even in airplane mode (with wifi subsequently turned back on) and behave the same way whether you are in your home country or abroad - ie no roaming charges.
These are often carrier specific (at least in my experience).