Hi there,
I double checked different scenarios, there is not a miminum length defined but there is a max length attribute.
This would suggest some misconfig or other issue occuring down to your particular deployment.
I would recommend checking you have a 3 digits label for you network VRU in the ICM config and if that is the case I would raise a TAC SR, providing gateway, GK and CVP logs, GW with RAS and ISDN Q931 debug turned on.
Regards,
Riccardo