It becomes a burden when parents tell their adult children that it is their duty to carry it - "We are your burden and you must take it up, and phone and visit us ten times as often as you have done, and do things for us in your very rare time off, whether you want to or not, even if we spend your whole visit going over again how selfish you are."^ Before that they may not have seen it as as something unpleasant that they ought to be doing, just hadn't really thought about it at all. They were encouraged to go out into the world and conquer it, and they are doing just that.
If parents have a life of their own, with hobbies, places to go, friends to see, they are less aware of the exact date and time that their children last contacted them. Because of this, when they do visit or phone, they are not greeted with a reproach for staying away - so the meeting is a pleasant one. Parent can chat about what they have done, hear about what child has done, be pleased about good news, and both parties enjoy it.
Result = child goes away happy, is motivated to make time to come again soon. There is scope for a phone call a little later, with more news about some ongoing situation, from either one of them. Not an hour-long chunk out of a child's all-too-short free time, but an update on things, and perhaps an arrangement to meet soon. As someone has said above, phones have outgoing connections as well as incoming ones. an "elderly" person, unless their faculties are impaired, is capable of ringing - and also of making it a fairly short call.
You catch more flies with honey than with vinegar.