One fulfills a Misva by visiting his father or mother, as this brings the parent joy. The same is true of phone calls or letters. Since the parent enjoys hearing from the child, this fulfills the Misva of "Kibbud Ab Va’em" (honoring parents).
The question arises as to whether one whose mother offers to serve him, such as if she offers him some cake or coffee, may allow her to serve him. Since the child is obligated to respect the mother, is it permissible to accept services that the parent wishes to perform for the child?
Hacham Ovadia Yosef, based on the ruling of the Shulhan Aruch (Yoreh De’a 240:25), writes that the child is permitted to accept this service, but should first respectfully ask the mother to forgive her honor. A child should not ask his mother to prepare or serve food for him, but if the mother offers food, he should ask her to waive her honor and may then accept the service.
This applies also if the father wishes to perform a service for his child. However, Hacham Ovadia writes (Halichot Olam, Parashat Shoftim, p. 148) that an exception is made if the father is a Torah scholar. Even if he is not a leading Torah sage, but is a Rabbi or full-time Torah student, the child should not accept services from the father unless the father insists. Whereas in other cases it suffices for the child to request the parent’s Mehila (forgiveness), in this instance, the child must refuse unless the parent insists on performing the service for the child.
If one has a choice between accepting a service from his father and accepting a service from his mother, it is preferable to accept the service from the mother, since she, like the child, has an obligation to respect the father.
Summary: If one’s parent offers to serve him food or drink, he may accept the service, but should first respectfully request that the parent waive his or her honor. If the father is a Rabbi or full-time Torah student, the child should refuse the father’s offer to serve him until the father insists on extending the service. One should not request a service from his father or mother.