Errors during Autodiscover procedure on Microsoft Exchange 2016

Question

I am working on IP phone software which connects to Microsoft Exchange Server via autodiscover procedure and works with Exchange data.

I am using Microsoft Exchange Server 2016.

At the present moment I have a problem with figuring out on which URL should I sent the auto discover request.

The request is the following:

<?xml version="1.0" encoding="UTF-8"?>
<Autodiscover xmlns="http://schemas.microsoft.com/exchange/autodiscover/outlook/requestschema/2006">
<Request>
<EMailAddress>user@domain</EMailAddress>
<AcceptableResponseSchema>http://schemas.microsoft.com/exchange/autodiscover/outlook/responseschema/2006a
</AcceptableResponseSchema>
</Request>
</Autodiscover>

I've tried several possible autodiscover URLs:

1) If I use URLs like described in: https://msdn.microsoft.com/en-us/library/office/jj900169(v=exchg.150).aspx

for example:

https://autodiscover.domain.com/Autodiscover/Autodiscover.xml

where 'domain.com' is our domain,

then I get 200 OK HTTP Response but with 601 Error inside:

<ErrorCode>601</ErrorCode>
<Message>Provider is not available</Message>

2) I checked the logs of Test E-mail AutoConfiguration utility which performs successful autodiscover and figured out the URL which it eventually uses. Its a bit strange because it includes user name in it and has two dots .. after it (which seems to be wrong format):

https://exchange.server.address/Autodiscover/user..domain/Autodiscover.xml

I tried this URL (with one dot only) and got 404 Not Found HTTP Response though Test E-mail AutoConfiguration succedded with this URL (according to its logs).

Also I tried the same URL but without user and domain:

https://exchange.server.address/Autodiscover/Autodiscover.xml

And get the same 601 Error as in p.1)

The questions are:

1) What might be the reasons of these two described errors?

2) How should I figure out which URL to use for autodiscover for my case?

Thanks in advance.