Anything that can receive SIP that's open to the public internet is going to get scanned, probed, and targeted.
The CUCM will ignore requests not destined for it that don't match a trunk or a registered client, but, that's not a good idea to expose it to the internet. It is likely to expose it to unnecessary traffic or resource usage, thus the use of a session border controller or some other proxy or whatever. The remote gateway is going to receive these requests but as you note, if you want to secure the two things, then you need to do so via some VPN tunnel (or equivalent "SD-" buzzword product), or firewall using an external firewall or access control list.
In the case of a gateway like the Expressway which is designed to allow B2B and unsolicited traffic, you can use call processing rules or call processing language on the device to tell it which traffic to match, and which to not. This requires you to be somewhat specific, and still has a processing impact on the appliance, but it has been fine in my experience.
