In a telecommunications system, in which a connection comprises a part with an interworking function at both ends, a channel is allocated to the connection between the interworking functions. The required channel capabilities may vary during the connection, whereby the channel capabilities should be changed. When the first interworking function detects that a channel capability must be changed, a first message, which indicates the desired capability change, is transmitted (2-1) to the second interworking function; and the channel capability is changed (2-3,2-5) into the desired one at both ends of the part.