One might have a filial responsibility derived from one's role as adult child to care for one's aged and infirm parents, and to do things for them like help them do their shopping. Such filial responsibilities create moral obligations which exist whether or not the agent has explicitly made a promise to the parent to, say, take her shopping on a particular day. These sorts of special responsibilities, filial obligations, are associated with the role occupied by the agent, being someone’s adult child, and one does not choose to enter this role.
An adult child with an aged and vulnerable mother may, of course, also explicitly promise to take their morther shopping on such and such a day. Here in the act of making a promise one has both explicitly acknowledged one's special moral responsibility, which pre‑existed the act of promising, and creates a right on the part of the parent under which she can demand that you fulfill what you have promised to do. The act of promising, in this case, also functions as an ascription of responsibility to a particular agent at a particular time, and this might be needed, for instance, in cases where there is more than one child who shares a filial responsibility towards the parent. The voluntaristic aspect in such cases concerns only the ascription of responsibility (i.e. volunteering to assume a shared responsibility on a particular occasion), and the creation of a correlative right. However, the content of the responsibility itself, the filial obligation to render assistance and care to aged parent who is in need of it, derives not from the promise, but from the vulnerability of the parent and the child's capacity to satisfy her needs, and perhaps also in this case from the notion of gratitude and reciprocity, (one can legitimately question whether an adult who was abused by his parent as a child owes that parent any special filial obligations).
The moral claim of the parent on the child's assistance pre‑exists the act of promising, so that act does not create the obligation, but only acknowledges and specifies it. If I make a promise to my mother to take her shopping, then if I fail to do so, I have both wronged her and have acted wrongly. However, I may still act wrongly if I fail to do this for her, even though I have not explicitly promised her that I would. I may act wrongly towards her if I understand that she is vulnerable to being stranded in her apartment because she is fearful of going out alone, and that she has no one else to turn to who will escort her to the department store. Under such circumstances, the adult child of an aged and infirm parent would have a filial responsibility towards her mother, even when she does not explicitly consent to it. The act of explicitly making a promise discursively legitimizes the underlying moral responsibility, but it does not create it.