We have an older Mitel 3300 using predominantly Mitel 5212 phones. We have a dedicated VLAN for voice traffic, which is tagged on our switches alongside whichever VLAN is providing the network for the computers. We are having a strange problem in which when a phone call is made, one side is unable to hear the other (in reverse it is fine). After a few seconds this will sometimes resolve itself, or they will need to hang up and try again. On the retry both sides can usually have full conversation as normal. This happens randomly throughout the day to different number / handsets that are plugged into various switches throughout the organisation. We have had a look at the switches and they don't have any obvious errors / are not reporting they have maxed out on throughput.
We have had our support engineers look at the phone system and they cannot spot anything obvious with regards to the fault.
Any clues as to where to start diagnosing this.