VOY is quite common in Roman epigraphy an means usually VOTA (promises/vows) so VOT MARTI would be 'vows to Mars'. When it is used in inscriptions it is usually used to indicate that a vow has been made and is now being redeemed. e.g. VSLM on altars, means that the vows are willing redeemed, as the gods merit it. I.e. The gods have kept there part, you keep yours.

An alternative are the celebrations on the accession day of the emperor, usually after 5, 10 and (lucky few!) 20 year. which usually read V X mult XX or variation thereof. I.e. we are celebrating 10 years, and renewing the vows for twenty.

In this context your VOT MARTI objects may be dedications at a shrine to Mars. But it is hard to say without knowing more about the context or the object it is found on.