Could be that they have one service and the other voltage is obtained from a transformer.
If the service is labeled 120/240 volts it might be a single phase service. If it is labeled as a 240/120 it might be a 3 phase and if it is it has a high leg delta.
If the voltage is labeled 120/208 it might be a single phase but if labeled 208/120 then it might be a three phase wye.
If I were you I would establish my service voltage before doing any estimates as the price for material of one voltage is different than for the other.