"RTCS bind IP err" resetting phone only happens with VLAN tag set

[lostinsip]

Hi

I have 5 Mitel 5212 phones which have been running fine in SIP mode on a FreePBX for some months.  The phones and the PBX have been running on the native/untagged VLAN (192.168.1.x) but now with a new switch with a voice VLAN capability I thought I would set up a tagged VLAN (tag 20) which is 192.168.20.x for the phones. 

In the web setup page of the phone I set the VLAN ID from -1 (the default and i am assuming this is untagged) to 20 and saved the settings.
The phone got an IP address from the router in the correct range (192.168.20.11) via DHCP and popped into the DHCP list on the routers diagnostics page for the DHCP server. The phone  went through all the normal setup displays but finally displayed "RTCS bind IP err restarting phone" it then reboots and carries on doing so indefinitely until i do a reset via the keys on bootup.

I have trawled about and have no idea if the issue is a setting in the phone i need to amend if using tagged VLAN or if I need to do something in the switch (although the phone is getting an IP address so I am assuming able to access the router)

There are settings for VLAN Priority and tos VAlue and PPPoE which I have not changed I do not know if those are relevant

here is a clip of the web settings screen off the endpoint https://imgur.com/6AqECkA

If i remove the VLAN tag it will hapilly go back to working on the native untagged VLAN on the same switch and router.

Any ideas gratefully received

Thanks

[Tech Electronics]

lostinsip,

Do you have routing setup between the voice vlan and the native vlan that the phone system is probably still on?

Thanks,

TE