The invention relates to a method and a communication system comprising at least one first network element connectable to a second network element. At least one of the first and second network elements use one of two or more selectable modes for communicating with another network element. One or both of the network elements, or a third network element, which is connectable with the first and second network elements, performs a mode selection procedure for selecting the same mode for bidirectional communication between the network elements. The modes preferably are different codec types, channel-coding schemes, or radio interface protocol types. The first and/or second network elements may be portable terminal equipments and the third network element preferably is a support node or a means providing a support function such as a CSCF. The mode selection ensures use of one and the same mode, such as a codec, in uplink and downlink directions enabling IP telephony in UMTS using SIP protocol.