As far as I know no free service exists that allow you to place call to standard phone networks (cellular or landline). You have to subscribe to an operator service like Skype as you named or other standard VOIP providers like Switzernet, Telcopack, Sipcall, Peoplephone, etc
There are none that I know of. However, UPC's landline isn't free either - you're either paying for the calls or paying Fr.10/month for calls within Switzerland. You could get those inclusive calls taken off your account and use the Fr.10 to fund an account with e.g. Skype.
Like many, I think it's a shame that UPC is discontinuing their phone app especially as they've finally got it working reliably.
edited on 28-04-2017 15:16
We kindly remind you that the UPC Phone App is available until 01.05.17. Next Tuesday, the App will be taken down from the app stores and the information on our website deleted. You may continue to use the app till August 31, 2017.
We still owe you an information: Unfortunately, we are not able to give you access to the SIP data. Sorry for that. Should this change at a later moment, you will be informed accordingly.
Thank you for your attention. Wishing you a nice weekend.