The tech support guy misinformed you. An Obi will support more than one phone on each PHONE jack. The exact number depends on the characteristics of the phones and there have been some, not totally confirmed, reports that long runs of house wiring can have an impact.
You should be fine with the base station plus the two corded phones.
See
http://en.wikipedia.org/wiki/Ringer_equivalence_number. OB1202 is specified as Max Ring Load = 5 REN.