I am pretty sure you will be charged for it, whether you get it through Unimax or an Avaya BusinessPartner. Like Ken Means said you need to have the software feature bit 17-Database Retrieval turned on for each remote system. If they are newer releases of software this is usually turned on.
Also, like Ken Means said, the Avaya Modular Messaging S3400 with either a Microsoft Exchange message store or Avaya's own MSS Linux message store is the newer messaging platform that replaces the Octel 250's. It has either the Aria telephone user interface or the Audix telephone user interface or both.