This Recommendation describes the voice-band data (VBD) operation of Voice-over-Internet Protocol (VoIP) gateways and media gateways. The term "VBD" refers only to the use of suitable voice-band codecs for the transport of data payloads via RTP. The VBD procedures described in this Recommendation shall apply to VBD-only capable gateways. An ITU-T V.152 gateway only has the guarantee of interworking with another gateway if that gateway also supports ITU-T V.152. This Recommendation supports hybrid modes of operation; for example, a device may support a VBD and facsimile relay capability but not a modem relay or text relay capability. In this example of hybrid operation modem and text payloads are transported in the VBD mode, while facsimile payloads could be transported in the ITU-T T.38 facsimile relay mode or in the VBD mode. The negotiation of such hybrid sets of capabilities follows SDP and ITU-T H.245 mechanisms (clause 11). This Recommendation describes the default mechanism for transitioning into VBD mode via payload-type switching as described in clause 10 and the optional mechanism of utilizing the state signalling events (SSE) messages described in clause 11.