Server-to-Server IAX issues

I currently have two separate Asterisk servers linked together through IAX over a VPN connection. Currently, Server A can make calls to Server B with no problems, but whenever I make a call from Server B to Server A, the phone connected to A will ring for a split second, hang up, ring again, hang up, and repeat this cycle 3 or 4 times. Meanwhile the phone that placed the call ends up with a busy signal.

I have set autokill to no in iax.conf since initially the phone attached to Server B wouldn’t even ring.

I supposed it could be a NAT issue, but the servers are connected via VPN, and neither of them have firewalls running.

Has anyone seen this behavior before when using IAX?