How is the phrasal verb stand by used in a sentence? It should convey the meaning, "to be available and ready to act if needed or called upon".
This is a beautiful example of a phrasal verb! Above when Jim said:
Please stand by and I'll answer your question.
The meaning of the verb has something to do with waiting in an attentive manner. Although the main verb is 'stand' the speaker likely cares very little if the person stands, sits, leans, squats or any other type of positioning. This differs from an example like:
Please stand by the door.
In the second example, the intent is to specify the location of where to do the standing, and in this case standing by the door can not be properly achieved if the person sits or squats. Although both examples have 'stand by' in them, the first is an example of a phrasal verb, but the second is not.
I fondly imagine that "stand by" is military in its origins, and joined at the hip with the phrases "stand to" and "stand down". "Stand to" is a shortened version of the order "Stand to your arms", which is an instruction to pick up your weapons, in preparation for action. The British Army has long had a tradition, which was still current when I was in the Territorial Army almost 40 years ago, and for all I know still is, of standing to arms at first and last light, as these were the times an enemy was most likely to attempt a surprise attack. Once it was obvious that the wicked enemy was not going to attack, the order "stand down" would indicate that everyone not detailed as a sentry could relax their state of alertness and carry on with whatever they were doing. "Stand by" I believe to be an instruction to assume the intermediate state between these two -- action is not imminent, but neither is it possible to relax completely, as one might be called upon at any moment. Analogously, with new-fangled electronic equipment, the old-fashioned "on" and "off" states correspond to "stand to" and "stand down", but "standby" mode means that your electronical iskermoffit is in an intermediate state where it does nothing useful, but continues to draw power. It is, however, poised to swing into action at any moment. The use of the verb "stand" in the phrase "stand to your arms" might seem odd, as "stand" seems essentially static, but the order suggests motion. This reflects the Naval usage of "stand" as a verb of motion, as in the signal "you are standing into danger" (Inernational Code Signal Uniform). The Naval usage still retains an element of stasis, in that it implies holding a steady course.