DEFINITIONS:
• An actual "grave worshipper":
A person who worships the inhabitant of the grave by intending worship by his call i.e. praying to him, by believing the inhabitant is worthy of worship, or that he shares attributes that are exclusive to Allah. And that he believes the inhabitant of the grave is independent and can benefit or harm without Allah.
And also carrying out acts like prostrating to the grave with the intention of worship. Or if you want to take it literally, someone who worships the structure of the grave!
• Pagans of Makkah:
Worshipped idols to get closer to their ''main God'', believed their gods were independent of Allah and could intercede without the permission of Allah, also believed Allah had daughters, and Allah fathered children such as the angels, and that the smaller Gods could override the decision of Allah. They also believed one god alone could not respond to the prayers of humans so other gods were needed and capable of fulfilling what (they believed) Allah fell short in. Ultimately attributing Divine Power to other than Allah.
• A Muslim Muwahid:
A person who believes in the Oneness of Allah, with all His names & attributes. Affirming that Only He has the right to be worshipped, and ONLY He creates & destroys, truly benefits & harms, gives life and takes life & sustains the universe and everything in it, and that NO creation has a share in this, and no creation is worthy of being worshipped. Ultimately attributing Divine Power to Allah ALONE.
• A Muslim Muwahid making a 'call':
A PERSON WHO BELIEVES ALL THE ABOVE who THEN simply makes a call which is obviously NOT a call of worship, to someone who is alive, in the grave, present, close, faraway etc. because he believes Allah gave him (the one being called) the ability to hear the call. The person whom the call is made to is believed to be NOT worthy of worship, and neither can help at all without the permission of Allah.
For example help can come in the form of the one who is being called e.g. a Prophet in the grave - supplicating for the health of the individual requesting/calling - as the one requesting believes the Prophet is alive and can hear by the Will of Allah.