Please note X is not an acceptable wildcard that you use at the GW level, . is the one you should use instead of X. Since you have not mentioned that how does your internal dial-plan within CM looks like, I am assuming that Telco will send starting digit as 5 or 6 followed by any two random digits and you will match your dial-plan based on those 2 digits.
So below rule will match any number starting from 5 or 6 followed by 2 digits and will convert it to 4 followed by the two digits. I have used 4 just as an example, you can use the prefix as per your dial-plan or change accordingly.
rule 1 /^[56]\(..\)/ /4\1/ type any subscriber
You really need to look at below document in order to understand how all this work to effectively manage your dial-plan.
https://supportforums.cisco.com/document/31816/custom-made-guide-translation-rulesprofiles-and-some-common-uses
Use below command on the router to test your rules
Note: test voice translation-rule <rule number> <number>
Regards
Deepak
