I successfully registered a Cisco 7821 IP phone on FreePBX 17, and it was working perfectly. Later, I opened the phone’s XML configuration file and accidentally changed the 101 tag to a text name like Ahmad.After uploading the modified XML file and restarting the phone, it completely failed to register. Realizing I made a mistake in the XML config, I changed it back to 101, re-uploaded the file, and rebooted the phone again. However, it still did not work.To fix this, I flashed two different firmware versions onto the phone, but the issue persisted. Interestingly, when I tested this exact phone with the same XML file on a different FreePBX 17 server, it registered and worked flawlessly.This led me to think the issue might be with the extension itself on the current FreePBX server. So, I deleted extension 101, recreated it from scratch, and tested a completely different phone on it. The new phone worked fine. Finally, I concluded that the problem lies within the current FreePBX server or the underlying Debian OS, as it seems to be caching the incorrect XML data or old registration state, and it needs to be cleared. I have tried many troubleshooting steps, but nothing has worked.What should I do now to get my original Cisco 7821 phone working on extension 101 again?