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.

1. 100 Trying

2. 180 Ringing

3. two-way voice path

4. Connect ACK

5. two-way RTP channel

This is the first thing that must be verified when troubleshooting the issue of phone status showing rejected in Cisco UCM 1 .  Auto-registration allows new phones to register with Cisco UCM without manual configuration 1 .  If auto-registration is disabled, the phone will not be able to register and will show a rejected status 1 .

The other options are not the first things that must be verified when troubleshooting the issue:

B.  whether the Initial Trust List and Certificate Trust List files on the phone are correct is not the first thing to verify, but it may be a possible cause of the issue if the phone has an ITL file from another cluster that prevents it from registering with Cisco UCM 2 .  To resolve this issue, the ITL file needs to be deleted from the phone or exchanged between the clusters 2 .

C.  whether the phone is in the correct VLAN is not the first thing to verify, but it may be a possible cause of the issue if the phone is not in the same VLAN as the Cisco UCM server or cannot reach it due to network issues 3 .  To resolve this issue, the network connectivity and VLAN configuration need to be checked and fixed 3 .

D. whether the phone’s MAC address is correct in Cisco UCM is not the first thing to verify, but it may be a possible cause of the issue if the phone’s MAC address does not match the one configured in Cisco UCM. To resolve this issue, the MAC address needs to be corrected and updated in Cisco UCM.