A method performed by a first UE. The method includes: the first UE sending via a first signaling protocol to a network node a service capabilities request, the service capabilities request requesting service capability information for a second UE; the first UE receiving from the network node a response to the service capabilities request, the response rejecting the service capabilities request; and as a result of receiving the response rejecting the service capabilities request, the first UE sending to the second UE, via a second signaling protocol other than the first signaling protocol, a service capabilities exchange invitation requesting an exchange of service capabilities.