The phrase 'to be at the beck and call of' OED does not record earlier than 1875. And with a beck ye shall me call. See Wyatt's miscellaneous poems . As SB points out, absence cannot be imprisoned, so a logical reading of this line is hardly possible. It is the necessity of conveying the ideas in a limited space that creates the compression of thought.
Imprisonment calls up the opposite idea of liberty, which the youth enjoys.
But the pain and suffering is caused by the loved one's absence and infidelity, which metaphorically imprisons the poet in the dark world of his own tortured reflections. Liberty also carries the idea of wantonness and libertinism , which is at the forefront of the poet's mind. And patience, tame to sufferance, bide each check, 7. Several interpretations are suggested of this line, which all tend in the same direction because of the basic meaning of the words. And in addition my patience will school itself to permit your charter, and I will endure each restraint you impose on me'.