In environments with poor WAN conditions—including packet loss, delay, and jitter—Cisco documentation consistently recommends codecs that are loss-tolerant, resilient, and designed for low-bandwidth or impaired networks. Among the available options, iLBC (internet Low Bitrate Codec) is specifically engineered to handle significant packet loss without producing severe audio distortion.

Cisco Collaboration design guides describe iLBC as a codec that uses independent encoding of each speech frame, allowing audio to remain understandable even when multiple packets are lost in transit. This is different from predictive waveform codecs like G.711 or G.729A, which depend on sequential packet flow. When packets are lost in those codecs, audio becomes choppy or unintelligible.