I assume that it has to be "live", and that recordings for later playback would not be acceptable.
My thought is that you might be best served with a conference call solution. being a single voice channel it doesn't require more than a standard phone line. It also gives you control over who can access the meeting, and also could provide participation for those accessing remotely (via speaker phone).
If this is to be a regular occurrence, why not invest in a phone line to the hall? You can always have the line locked and the equipment removed when not in use.
Of course, webcasting is a possibility, if 2-way participation is not required, and there are a number of software solutions available to create the streaming media content.
However - and it is a big however - you would really need a broadband connection to manage the data required, and with neither phone line nor broadband connection, you would be struggling! 3G is feasible but expensive.
The range of microphones is limited (for either scenario), however - most systems are designed for table top solutions (in my experience), though I'm sure that there are other solutions available - you would need to investigate this depending on the number and arrangement of the participants.
SD