Route groups are only used for calls that are destined to a number that are outside of your CM system. As the branch office is part of your CM that doesn’t need to pass by the route patterns, route lists and route groups construct as they are intra cluster calls.
Most of, if not all of @Enes Simnica answer seems to derive from an AI search as it IMHO doesn’t contain relevant information. For one as the offices belongs to the same CM system there is no such thing as Publisher dial plan that would differ from the Subscriber dial plan as that both are part of the same CM and therefore share the same dial plan. Secondly as outlined above the RP, RL and RG construct isn’t applicable to your described call case. Thirdly inter cluster trunks are not applicable for calls which are within the cluster, those apply when you have multiple CM clusters that you’d want to pass calls in between.
This is controlled by the calling search space assigned to the directory number and device. If you cannot make calls to number(s) that would indicate that the directory number is in a partition that isn’t visible in either of the mentioned CSSs.
What result do you get if you do a DNA for a call from a device/directory number at HQ to a directory number at the branch office?
