In most, if not all, national dialing plans there is no possible overlap with emergency services. For example in the United States, 911 has no possible conflicts - that NPA is reserved - and toll calls require a trunk prefix of one (1). When you dial 911 on a landline in the USA there is nothing else you can dial so the CO switch stops accepting input and routes the call immediately.
The same approach is typically used in CUCM call routing design: build specific Route Patterns (or Route Filters if that’s your thing) that do not accept additional digits after the emergency services pattern. In your case, by the time the user dials the third digit “911” CUCM should ideally have only one possible matched pattern: your CER CTI RP. This is possible because both the NPA and NXX must start with 2-9. As such “911”, “9[2-9]XX[2-9]XXXXXX”, and “91[2-9]XX[2-9]XXXXXX” are all non-overlapping. In contrast, “9911” may overlap in seven-digit local dialing areas or ten-digit areas that include an NPA starting with nine. The typical approach is to use Urgent Priority, forcing CUCM to not accept additional digits once that fixed-length pattern is matched.
Reminder that Kari’s Law now mandates support of straight “911” in the USA; you cannot require the user dial “9911”.
