It could be from your ISP renewing your lease. It makes no difference if the public IP changed or is the same. The lease periodically gets renewed.
I am a little confused, if you are losing registration, you should not be able to make outbound calls as well as receive inbound. If it is registered and making outbound calls is OK and you are sure of your router port forwarding settings, I would first start by disabling (if set) any type of custom ring settings (distinctive ring, simul ring etc.)