We were tasked to look into a MiVoice Business controller in the UK that had been recently rebuilt from backup. Incoming or outgoing calls to the controller turned any calls into almost inaudiable noise.
Checking the ISDN forms against best practice it looked like nothing had been missed or mis-set, it was restored from a backup after all.
The key factor was eventually realised that this Mitel controller was freshly rebuilt with software reinstalled – the country was not set correctly! It was set to North America rather than United Kingdom.
The country setting of North America had messed with the codec/tones of the ISDN, so was producing the horrible noise on the phone line.
The Mitel controller during an out of hours window was set to United Kingdom in the country settings, system reset, and back-up restored.
The Mitel controller was back online, a call tested, perfectly clear audio.