The benefit we derive from this phrase in this Hadeeth  is that it is an obligation to respond to your parents if they call on you while you are in prayer, but with the condition that the prayer is not an obligation. If the prayer is an obligation, then it is not permissible to respond to them, but if it is a Sunnah prayer then respond to them.
If however your parents are amongst those who weigh issues out correctly, and if they knew that you were praying when they called you they would excuse you, in this case you can signal to them that you are in prayer, by making a noise or saying Subhaan Allaah in a loud voice or raising your voice with the Ayah that you are reciting or a supplication that you are reading until the caller understands that you are in prayer.
If you know that your parents, your mother and father, are understanding and would excuse you if they knew that you were praying, then just indicate to them that you are praying.
For example, if your father approaches you and calls you by name while you are praying the Sunnah of Fajr, if he is an understanding man who would excuse you, then indicate to him that you are in prayer, say Subhaan Allaah, raise your voice in recitation or Du’aa or the Dhikr that you are reading so that he excuses you.
If your father however is amongst those who would not excuse you, he wants his say to be the uppermost, then cut off your Salaat and speak to him. Likewise is the case with your mother.
As for the obligatory prayer, then do not cut it off for anyone except if it is a necessity. For example, if you see someone who is in danger, such as someone falling into a well, the sea or a fire. In such a case cut off your obligatory prayer, other than such cases [i.e. emergencies] then do not cut off the obligatory prayer.
 The Shaykh mentions this in his comments to the Hadeeth of Abu Hurayrah that was collected by both Bukhaaree and Muslim where Jurayj was called upon by his mother on three occasions while he was in prayer and he chose not to respond, she eventually supplicated against him. The full text of the Hadeeth will be translated in another article shortly, inshaa Allaah.