Thank you all for the good tips. I found the issue! Luckily not an issue with Kamailio after all. The issue is that Linux enables reverse path filtering by default. I tried to substitute Kamailio with a simple Python server, and I noticed that it wasn't showing me any output either. So I started digging around in the Linux settings and found that we needed to disable reverse path filtering.

The Python script used to substitute Kamailio:

import socket
sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
sock.bind(('0.0.0.0', 5060))

while True:
	data, addr = sock.recvfrom(1024)
	print "Received message:", data

More information about reverse path filtering:
https://www.tldp.org/HOWTO/Adv-Routing-HOWTO/lartc.kernel.rpf.html

I guess we missed this setting during migration. If it helps anyone in the future, to disable remote path filtering (at your own risk, obviously):

for interface in /proc/sys/net/ipv4/conf/*/rp_filter; do
echo 1 > ${interface}
done

To be clear, the mhomed setting is not mandatory and in our case not desired.

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