Which it is the oxidation state of indium and phosphorus?
I've estimated net atomic charges in rocksalt type InP by simple HF calculation in CRYSTAL06 in random basis.
Lattice parameter of 5.54 A is taken from:

K Kabita et al. First principle study on pressure-induced electronic structure and elastic properties of indium phosphide (InP). Indian J. Phys. (2015), DOI 10.1007/s12648-015-0701-0.

Results: http://pastebin.com/zdb8js8Z

Total atomic charges are 47.2861139, 16.7138861, so partial charges are:

In: +1.71
 P: -1.71


Given that oxidation state is an hypothetical charge that an atom would have if all bonds to atoms of different elements were 100% ionic, the answer is probably better estimated as +2/-2, not 1.