How would I receive a call if he is not able to call me or communicate
with emergency personal?
There is no ironclad method. The law does not specify how authorities should determine contact information for next of kin. As a practical matter some of the better methods (few people use all of them) are:
Create an "emergency contact" business card and put it that person's wallet and/or purse. Put another such card in a prominent place in their home like taped to the wall next to their phone charger or under a magnet on the refrigerator or on a cork board in the house. Make sure that the person has their own ID and medical insurance card and a card containing any medical alert information (e.g. drug allergies and blood type and religious objections to any particular sort of treatment) there as well. A "wallet biopsy" is standard operating procedure for first responders when the identity of a person suffering an emergency is unknown.
Enter your name as an "in case of emergency" (ICE) number on that person's cell phone. You can also set up their cell phone to authorize you to locate it with an app. More crudely, you can put a sticker that says "emergency contact" with your name and phone number physically on the outside of the person's phone. The mechanics of putting emergency information in a phone can be found at this insurance company website.
If they have a medical alert or home security system or medic alter bracelet, have them put you as an emergency contact for that.
Put a medical power of attorney naming you as an agent (if the person is willing to execute one) together with your contact information in the patient file of all of the person's medical providers such as a treating primary care physician, home health care person, etc. Keep the number for these providers on hand so that you can call them to ask if there is any news or appointments have been missed.
Have the person list you as an emergency contact in places that keep records of one such as an employer, membership based gym, a college or educational institution where the person is taking some classes, and their nursing home or assisted living center (if any).
Provide your contact information to (and get contact information from) neighbors, landlords, financial advisors, accountants, lawyers, and family members of the person who are likely to be contacted and ask that they let you know if something happens. Spend at least a little time with as many of them as possible in person, when you are in town, to the fullest extent possible. Become friends with them on social media and interact with them every once and a while in that context. Put them on your Christmas letter list. Share your excess tomatoes and strawberries with them. Send them little thank you notes and gifts when you learn that they did something nice for the person like helping them shovel snow or trimming the shrubs on their shared property line. Little courtesies create a moral impetus on their part to take the trivial effort of calling you to let you know that something is up when it happens.
Have a local contact you can have look into the situation if you are unable to reach the person at the usual times, ideally someone with a spare key and security codes to the house and/or apartment building or gated community. If there is no one who can do that, local law enforcement can be asked to do a "welfare check" on the person.
Have a copy of a will, power of attorney, or HIPPA release that allows a third party to corroborate your connection to the person and be in a position to tell someone local where the original will is located. It is also good to keep a log or journal of your contacts with the person so that you can demonstrate that when they go missing that it really is an unusual and concerning event and to demonstrate that you are in regular contact with the person.
For example, I have a client who died this week while his emergency contact person was visiting family in another country. When she was unable to reach him, she checked with his doctor, learned that he had failed to show up for a medical appointment scheduled for earlier that day, and then had her son who was still in town use a spare key she had given him to check in on my client, where her son had the misfortune of discovering the deceased client.