A retired participant recalled to duty in the Agency under subsection (a) of this section or reinstated or reappointed in accordance with section
2051
(b) of this title shall, while so serving, be entitled, in lieu of the retired participant’s annuity, to the full basic pay of the grade in which the retired participant is serving. During such service, the retired participant shall make contributions to the fund in accordance with section
2021 of this title.