1. Recommended that you limit traffic to and from your phones ONLY from RingCentral IP addresses. You may need to enage the help of an IT professional to do this. The IP ranges you should be accepting traffic from are 184.108.40.206/22 and 220.127.116.11/22
2. To help with this issue RingCentral has recently implemented encrypted SIP registration or SIP TLS. This encrypts all communications back and fourth between RingCentral and your phone. This eliminates the ability for anyone on the internet to be able to just log in to your phone. You can ask to have this enabled by calling in to RingCentral support and asking them to enable this feature. Note: This conversation was created from a reply on: Why am I receiving calls from no one?.