There was no endpoint listening

I am trying to interact with an SharePoint Server 2010 using webservice call, which belong to one of my customer.

When I make a webservice call to a subsite http://myserver/subsite1/subsite2/subsite3 it is working fine. But when I try to call the same webservice on the myserver I am getting the following error:

There was no endpoint listening at http://myserver/_vti_bin/webs.asmx that could accept the message. This is often caused by an incorrect address or SOAP action. See InnerException, if present, for more details.

I am unable to understand the reason of this error as the same webservice is working on subsite.
I do not have access to web.config or any server side configuration.

Someone please tell me what could be issue here and how to resolve this issue.