The invention related to handling core network entities of a radio core communications network comprising a first network entity and a second network entity. The said second network entity differs from the first network entity in that it is capable of processing interworking between messages exchanged with said radio core communications network and messages exchanged with an IP multimedia subsystem. In other words, one network entity is capable of performing inter-working while the other one not. The first and/or second network entities are provided (S200) with configuration information indicating a relationship between the two network entities. The invention then foreseen routing (S300) messages relating to calls established between the at least one user terminal attached to the entity non capable of inter-working and a further user terminal attached to said IP multimedia subsystem according to said configuration information.