Alok is definitely right and the deployment guide is what I would also have linked.
There are a some threads around that here in the forum, its worth searching/browsing for them.
Some additional thoughts. Most scans are done today by sip/udp. New VCS setups have that disabled
by default as its not used in most TelePresence/Videoconferencing setups, so it would most likely not
harm you anyhow.
If you do not have an exposed isdn gw these calls just fill up your log and if you have an endpoint
which listens to that address it might get annoying if that rings all the time.
(that also happens with reachable endpoints on public ips)
If you have an ISDN gateway you definitly need to check how to secure it as it can generate high
costs if someone finds a way to dial out to premium numbers or expensive international destinations.
Besides that, what can affect you is that they might end up in some kind of (wanted or not) denial of service (dos)
as it might fill up your mcu or that some loop or many call attempts eat up your call licenses.
You should check why you get a call loop. Thats often caused by matching any search rules in both
directions, additional rewrites by transforms/searchrules, ...
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