Can I get DeviceId in Telephony subscription events?

What type of device are you using? Today you can only get device IDs for Softphone , Hard Phone devices via Device endpoint https://developers.ringcentral.com/api-reference/Devices/listExtensionDevices

Unfortunately , you will not be able to get deviceIDs for RC App or Web-phone apps .

The device id is reported in the telephony session event payload only for outbound calls. It is included in the event through the .NET SDK. Here is what I got in my test using the .NET SDK

// code snippet
...
var eventFilters = new[]
   {
       "/restapi/v1.0/account/~/extension/~/telephony/sessions"
   };
var subscription = await pubNubExtension.Subscribe(eventFilters, message =>
   {
      Console.WriteLine(message);
      ...
   }

// console 

{....,"parties":[{"accountId":"xxxxx","extensionId":"yyyyy","id":"p-a0d7bfa00bcb8z17efe18a620zabb6420000-1","direction":"Outbound","to":{"phoneNumber":"+1650224xxxx"},"from":{"phoneNumber":"+1720386xxxx","extensionId":"yyyy","deviceId":"802404926016"},"status":{"code":"Setup","rcc":false},"park":{},"missedCall":false,"standAlone":false,"muted":false}],"origin":{"type":"Call"}}}
----
{....,"parties":[{"accountId":"xxxx","extensionId":"yyyy","id":"p-a0d7bfa00bcb8z17efe18a620zabb6420000-1","direction":"Outbound","to":{"phoneNumber":"+1650224xxxx"},"from":{"phoneNumber":"+172038xxxx","extensionId":"yyyy","deviceId":"802404926016"},"status":{"code":"Proceeding","rcc":false},"park":{},"missedCall":false,"standAlone":false,"muted":false}],"origin":{"type":"Call"}}}
----
{...,"parties":[{"accountId":"xxxx","extensionId":"yyyy","id":"p-a0d7bfa00bcb8z17efe18a620zabb6420000-1","direction":"Outbound","to":{"phoneNumber":"+1650224xxxx"},"from":{"phoneNumber":"+1720386xxxx","extensionId":"yyyy","deviceId":"802404926016"},"status":{"code":"Answered","rcc":false},"park":{},"missedCall":false,"standAlone":false,"muted":false}],"origin":{"type":"Call"}}}
----
{....,"parties":[{"accountId":"xxxx","extensionId":"yyyy","id":"p-a0d7bfa00bcb8z17efe18a620zabb6420000-1","direction":"Outbound","to":{"phoneNumber":"+1650224xxxx"},"from":{"phoneNumber":"+1720386xxxx","extensionId":"yyyy","deviceId":"802404926016"},"status":{"code":"Disconnected","rcc":false},"park":{},"missedCall":false,"standAlone":false,"muted":false}],"origin":{"type":"Call"}}}

Unfortunately, there is no device id in any call stage for inbound call. However, I found a workaround solution below for you.

As soon as you get the "Answered" event of an inbound call, call the endpoint to read active calls for that extension. Set the "view" = Detailed to get the call detailed info. The device id is inside the call legs array at the leg with the direction = "Outbound". Some code snippet in Node JS here

...
subscription.on(subscription.events.notification, function(msg) {
    console.log(JSON.stringify(msg));
    console.log("======");
    if (msg.body.parties[0].status.code == "Answered"){
      readActiveCall(msg.body.telephonySessionId)
    }
});

async function readActiveCall(telSessionId){
  console.log("readActiveCall")
  try {
    var endpoint = `/restapi/v1.0/account/~/extension/~/active-calls`
    console.log(endpoint)
    var resp = await platform.get(endpoint, { view: "Detailed" })
    var jsonObj = await resp.json()
    for (var record of jsonObj.records){
      if (telSessionId == record.telephonySessionId){
        console.log(JSON.stringify(record))
        console.log("=====")
      }
    }
    console.log("+++++++++");
  }catch(e){
    console.log(e.message)
  }
}
// sample response
{
  "uri": "https://platform.ringcentral.com/restapi/v1.0/account/80964xxx/extension/6228832xxx/call-log/QE3d-cD9VKkHzUA?view=Detailed",
  "id": "QE3d-cD9VKkHzUA",
  "sessionId": "777389694016",
  "startTime": "2022-02-17T23:16:51.000Z",
  "duration": 86,
  "type": "Voice",
  "internalType": "LocalNumber",
  "direction": "Inbound",
  "action": "Phone Call",
  "result": "Accepted",
  "to": {
    ...
  },
  "from": {
    ...
  },
  "reason": "Accepted",
  "telephonySessionId": "s-a0d17932e8d6bz17f09f959f4z73533f0000",
  "legs": [
    {
      "startTime": "2022-02-17T23:16:51.000Z",
      "duration": 86,
      "type": "Voice",
      "internalType": "LocalNumber",
      "direction": "Inbound",
      "action": "Phone Call",
      "result": "Accepted",
      "to": {
        ...
      },
      "from": {
        ...
      },
      ...
    },
    {
      "startTime": "2022-02-17T23:16:51.000Z",
      "duration": 86,
      "type": "Voice",
      "internalType": "Sip",
      "direction": "Outbound",
      "action": "VoIP Call",
      "result": "Accepted",
      "to": {
        ...
      },
      "from": {
        "name": "Paco Vu",
        "extensionNumber": "11119",
        "device": {
          "uri": "...",
          "id": "802636634016"
        }
      },
      ...