what are the phone calls?
quick ones with a quick yesor no question? or long elaborate ones that require a lot of time?
i would say to them you are happy yo help them but it needs to be on your terms, not theirs - you are the one doing them a favour.
tell them the calls are becoming a nuisance and causing you stress and upset and inconvenience with their frequency
i would ask them to write down all the questions they want help with - to sit down and really think it through and say you will answer all on the list - ONCE, and that's that.
or ask them to send all questions once a week in an email - so you can respond at your convenience.
i would also just start being 'useless' ... be vague, forgetful, etc
also just be generally unavailable - always be out, or busy etc - can you say you have found another job and you cannot have your phone at work ... perhaps some volunteer stuff
i do understand their predicament - you are the only person they can turn to - but they are taking liberties and need to sort themselves out, get organised and get the information they need in one go - if necessary they should hire someone to speak to you and create a manual for the next person.