Synonyms for 'At Someone's Beck and Call': Graded by Power Dynamic

At one's beck and call = constantly available, subservient to another's demands.

Synonyms by Nuance

Tone	Phrases
Neutral/Descriptive	Available on demand, on call, at disposal, ready when needed
Formal/Professional	At the direction of, subject to the instructions of, accountable to
Subservient/Deferential	At beck and call, at command, obedient, compliant, submissive
Exploitative/Negative	Enslaved to, indentured to, at the mercy of, dancing attendance on
Idiomatic/Colloquial	Jump when they say jump, snap to it, on a string

Origin

Beck = silent gesture (nod, hand signal); call = verbal summons. Together: responsive to slightest signal.