I'm gonna guess the question is really asking about a BYOD provider since the question is posted here on an Obi forum.
That being so, I notice the major BYOD providers used here -- Anveo, CallCentric,
VoIP.ms -- do not show Nicaragua on their list of DID countries.
In the list dircom linked to, I see only one provider than shows BYOD devices, but their website is in Spanish so I couldn't poke around any further.
A note to normanc, look at nic2globe in the above list, see where it shows "Equipment:" and lists Cisco Systems, Grandstream, Sipura. That's sign they probably allow you to use your own equipment (Bring Your Own Device), all three are providers of devices similar to the Obi devices, with the big exception being the devices do not do GV.
You also might have better luck finding a provider on the BroadBandReports VoIP Chat forum.