I pretty much only use gold to buy outfits. Sometimes I'll spring on a chest for my guildies, though.
The only call worth making, other than the free one, is the three-star (15-phone) one. Every now and then, they'll lower the cost of those calls to 10 phones, but NG hasn't done that in about six months now.
As another pro tip, get the Google Opinion Rewards app. (I'm not sure if it's available for iOS.) You do super quick little surveys and build up Play Store credit, which you can use for in-app purchases.