VOIP POE Phone brings down Data network

Hi All, Long time lurker here

I am a jack of all trades sysadmin and use contractors to implement any projects as required

I have a POE Stack (VOIP POE Telephone system ) and Data Stack (PCs/Servers) which are connected to use a "one line to desk" option if required. This has been fine and working for years. On Wednesday i came into work and had to deal with "nothing is working!"

Looking at the network all PC / Servers connections are intermittent or have high performance. I started at the Windows end looking at DNS , DHCP, Domain controllers , AV etc..... but when nothing resolved the issues. I looked further into the switch/networking end.

I confirmed that nothing out of the normal was happening on the switch stacks CPU's or Logs but finally disconnected the LAG from the POE stack for the VOIP system to the Data stack. This resolved the issue instantly and reconnecting replicated the performance issue.

When i finally was able to chat to the network VOIP contractor that setup the system he was able to confirm that all the PCs and Servers wanted to connect to the network via what appears to be a single VOIP handset on a single port of a POE switch. When i disconnected the Phone and reconnected the LAG between the stacks the performance was no longer an issue. When i tried to reconnect the Phone in question to replicate the problem, it did not cause any performance issues at least for the 5 mins it was connected

The Phone is question was being used in to connect a PC to the data network which is now connected to main Data network without issue. The PC is completely locked down for a non admin users, so there should be no viral or malicious software

I hope this makes sense

I am still confused on how a single VOIP Phone can bring down an entire Data network

Any advice would be welcomed.