A method performed by a first IP multimedia system, IMS, node (15), for handling a network function in a communication network. The first IMS node (15) requests a first core network node (13) for at least a part of contact information of a network function instance, and receives a response from the first core network node (13) with at least the part of the contact information for the network function instance. The first IMS node (15) communicates with a second IMS node (16) or a core network node over a service based interface, based on the received part of the contact information for the network function instance.