Welcome to the RingCentral Community

This is the official NuGet package that we support: https://github.com/ringcentral/ringcentral-csharp-client

It is possible to specify the perPage parameter.

https://github.com/ringcentral/ringcentral-csharp-client/blob/master/RingCentral/Paths/ExtensionPath...

Sample usage:

rc.RestApi().Account().Extension().List(new { perPage = 1000 });

The maximum perPage allowed is 1000. If you have more records, you still need to handle pagination. Please refer to VB's answer for pagination handling.

Hi ,

Assuming you are using /account/~/extension API to fetch extension info ,  here is the response you will get 

{
  "uri" : "https://platform.devtest.ringcentral.com/restapi/v1.0/account/131074004/extension?page=1&perPage...;,
  "records" : [ 
//ext details
 ],
  "paging" : {
    "page" : 1,
    "totalPages" : 1,
    "perPage" : 100,
    "totalElements" : 8,
    "pageStart" : 0,
    "pageEnd" : 7
  },
"navigation" : {
    "nextPage" : {
      "uri" : "https://platform.devtest.ringcentral.com/restapi/v1.0/account/131074004/extension?page=2&perPage...;
    },
    "firstPage" : {
      "uri" : "https://platform.devtest.ringcentral.com/restapi/v1.0/account/131074004/extension?page=1&perPage...;
    },
    "lastPage" : {
      "uri" : "https://platform.devtest.ringcentral.com/restapi/v1.0/account/131074004/extension?page=4&perPage...;
    }
  }
} 

You can use "next" navigation page url to seek to next page with the list of extensions  .