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Could be a few different things.

Is this a new installation, or existing?
If existing, was it working before?

What kind of breaker panel do you have? Possibly GE or FPE?

A 240V circuit would not have a neutral.

This ALL depends on the POCO. MOST will NOT spend the $$$ to bring three phase into a residence. Why? Because it is simply NOT worth it, at least to them.
WHAT in a residence could possible need...

In a residence it is safe to say you will pretty much never see 208v.

If it's not a residence it's NOT DIY.

If a two pole breaker is used then what is there is likely legal, IF on a 15A breaker. Single pole breakers can ONLY supply 120v circuits.

Again, all moot points for the OP's situation.

FORGET that existing receptacle and call an electrician to run you a new 50 amp, 240v circuit.
That unit DRAWS 40 amps, so you will need a 50 amp circuit.

IMO this is NOT for anyone but the most...

This is just a correction so as not to confuse folks.
IN the US we use SINGLE phase for residential. Commercial can be either single ohase or three-phase.

Two hots off a single transformer is...