The phrase at someone’s beck and call means: always having to be ready to obey someone’s orders immediately.

The noun beck denotes a mute signal or significant gesture (such as a nod or a motion of the hand or forefinger), indicating assent or notifying a command.

This noun occurs, for example, under De Faunis & Satiris, in Batman vppon Bartholome, His Booke De Proprietatibus Rerum ¹, Newly corrected, enlarged and amended: with such Additions as are requisite, vnto euery seuerall Booke: Taken foorth of the most approued Authors, the like heretofore not translated in English. Profitable for all Estates, as well for the benefite of the Mind as the Bodie (London: Thomas East, 1582):

Some of them haue closed mouths in their breasts onely one hole, & breath and sucke as it were with pipes and veines, & these be accounted tonguelesse, and vse signes and becks in steed of speaking.

¹ The Franciscan encyclopaedist Bartholomaeus Anglicus (c.1200-1272) was the author of De Proprietatibus Rerum (On the Properties of Things), which was translated into English by John Trevisa (c.1342-c.1402), then edited and revised by Stephen Batman (c.1542-1584).

The word beck was originally a verb meaning: to make a mute signal or significant gesture. This verb is a shortening of the verb beckon.

These are, in chronological order, the earliest occurrences of the phrase at somebody’s beck and call that I have found:

1-: From A pleasaunte Laborinth called Churchyardes Chance, framed on Fancies, vttered with verses, and writtē to giue solace to eury well disposed mynde (London: John Kyngston, 1580), by the English soldier and author Thomas Churchyard (c.1520-1604):

He seru’s not one, but waites on all, that there would placed bee,
And bounde he is at becke and call, to hye and loe degree:
Now trudge and trott at one mans héeles, then to an other streight.

2-: From A siluer Watch-bell. The sound wherof is able (by the grace of God) to win the most profane worldling, and carelesse liuer, if there be but the least spark of grace remaining in him, to become a true Christian indeed, that in the end he may obtaine euerlasting saluation. Whereunto is annexed, a Treatise of the holy Sacrament of the Lords Supper (London: Printed by T. C. for William Cotton, 1605), by the English clergyman Thomas Tymme (died 1620):

He [= the Lord] is that mightie Captaine, who hauing vnder his gouernement many souldiers and seruants, hath them so at his commandement, that when he biddeth them go, they go: when he saith abide, they abide: and when he willeth them to do this or that, they obey his word. For paine, pleasure, griefe, ease, sicknesse, health, life and death, are at the becke and call of God, and do come and go at his appointment.

3 & 4-: From Salue Deus Rex Iudæorum. Containing, 1 The Passion of Christ. 2 Eues Apologie in defence of Women. 3 The Teares of the Daughters of Ierusalem. 4 The Salutation and Sorrow of the Virgine Marie. With diuers other things not vnfit to be read (London: Printed by Valentine Simmes for Richard Bonian, 1611), by the English poet Aemilia Lanyer (1569-1645):

3-: From To the Queenes most Excellent Majestie:

The Muses doe attend vpon your Throne,
With all the Artists at your becke and call.

4-: From The Passion of Christ:

Those deare Disciples that he most did love,
And were attendant at his becke and call,
When triall of affliction came to prove,
They first left him, who now must leave them all:
For they were earth, and he came from above,
Which made them apt to flie, and fit to fall:
Though they protest they never will forsake him,
They do like men, when dangers overtake them.

5-: From Heavens Treasury opened in a Fruitfull Exposition of the Lords Prayer. Together with the principall Grounds of Christian Religion briefly unfolded (London: Printed for R. Dawlman, 1645), by the English theologian and colonial leader Thomas Hooker (1586-1647):

The servants of the Lord […] are not withdrawing themselves, but ready upon every occasion to doe what the Lord requires Iob. It is said, that The Sonnes of God appeared before him; they are ever in his sight to give attendance to him, as a handmaid is at the hand of her mistres. So that of Isaiah, They cover their faces before the mercy seat, they cover their faces, in token of awfulnesse and reverence, and cry holy, holy, holy. They are ever before him as a dutifull servant at his masters beck and call.

6-: From Mr. John Arndt ² (that famous German Divine) His Book of Scripture. Declaring that every Child of God ought and must 1. Daily die to the old Adam, but to Christ live daily. 2 And be renewed to the Image of God day by day. 3. And in the New-birth live the life of the New Creature. Translated by R. M. K. (London: Printed by Mat. Simmons for H. Blunden, 1646):

Paul saith, 1 Cor. 2. The naturall man understandeth not the things that are of the spirit of God, for it is foolishnesse to him, and he cannot understand it: that is, he hath not the least spark of the spirituall light, tasting nothing of those things which belong to an heavenly, divine, and spirituall life, to which man onely was created, that being enlightned with spirituall light by the inward eyes of the soul, he might contemplate the presence of God, and his most sincere love towards him, might live with him, and depend on his beck and call throughout all his life.

² Johann Arndt (1555-1621) was a German Lutheran theologian.