I have only ever used the forwarding ON the iphone, for the past 4-5 years, with ATT and forwarding to my google voice number. As have many family members and friends when travelling to Canada, South America, Europe, Eastern Europe, Russia, Turkey, Bulgaria, and even south africa

Never had a charge for any calls, missed calls. NOTHING gets sent to voicemail, since it goes straight to google voice, and it hits THAT voicemail.

Now, I can't say that there is NO possible options where this won't work as designed, but as I've said I've used it for YEARS and never had a single issue.