If you don't want to create the wallet software for the ring these are two of the most famous wallets for Android (no bitcoin for iphone for now thanks to Apple), you can talk to them and see how to integrate:

• Bitcoin wallet (the guy already has some interfaces to pay with bluetooth and probably NFC) https:-//play.google.com/store/apps/details?id=de.schildbach.wallet
• Blockchain, very famous and nice wallet https:-//play.google.com/store/apps/details?id=piuk.blockchain.android