You are not sure what it means ~ how about THEY use the same secretarial service ~ co-located there or somewhere else ~ the magic machine behind that is called a PBX.Except that all three companies (Rossi's EON company, AmpEnergo and NAI Global [owned by the same man who founded AmpEnergo]) list this address as their primary business address with the state of New Hampshire.
Even if they were using a phone service, it is unusual to use the same number for separate companies (such phone services usually provide separate numbers to each of their customers so they can provide custom greetings for each customer).
And, it's obviously not a common number for the businesses in that building, since all of the other companies in the building have their own phone numbers.
My brother runs a machine shop, and at one point they created a new, wholly-owned subsidiary to build automated assembly lines. The new company was in the same building (which made sense since they were owned by the same people and the machine shop was making the parts to go into the assembly lines), but they had additional phone numbers created for the new company so that when they answered the phone they could give the appropriate company name.
Sharing the same phone number means they have to ask every caller which company they are trying to call, which is not very professional.
This suggests that both Rossi's company and AmpEnergo may only be separate companies "in name only". If this is true, it would be deceptive (but not necessarily illegal) to publicize their "deals" as between independent companies.
This is not unusual particularly where you do not have a high volume of "call ins" (like walk ins at a shopping center).
NO ONE CARES in those cases.
So, they share a phone number in New Hampshire. They seem to do business in Italy and Florida.