Recommended MSS value on ISP PE interface facing CE?

Hi All,
We are experiencing issues with desk phones authenticating to Microsoft Team, We had several troubleshooting with ISP and the action that they did to get this work is to set the TCP MSS value on their PE interface to 1460...... but the problem occured again after a few hours and we are seeing the same issue in where deskphone can't communicate to MS team.
Diagram/FLOW: https://ibb.co/hWXNR1W
From the packet capture on CE we are seeing 2 things.
1. RST packet coming from public network (unable to identify since we don't have control)
2. TCP session is working (3 way handshake) however client is sending "Client hello" however no server response.
Question:
1. Any idea why after changing the mss value to 1460 the deskphone connection works and suddenly stops after a few hours ?

1. By default the ISP is using the default MSS value which is 536Bytes. What is the recommended value for MSS on the ISP side ?

2. The only value that is being accepted from df ping is 1496. Is it possible that it is causing the issue? Since if the payload is 1460 + 20 + 20 for header the total is 1500 however the MTU allowed up to Microsoft peering is 1496?

3. Packet capture from CE router shows client sending "do not defragment" and the payload size of 1300+ so means there are still available for additional overhead.. Is there any possibility that additional overhead is added along the way which exceed to the maximum value of 1496?
   Thank you