OBI device with Nortel Meridian

[MirageAruba]

I am ready to purchase whatever Obi Device i need, as soon i figure out the below:
I have a Nortel Meridian with +30 Extensions at my Office, i have 3 Extension (30,31,32) that are available.
I have 3 Remote Offices that i need one extension for each

Remote Office 1 ( Extension 30 )
Remote Office 2 ( Extension 31 )
Remote Office 3 ( Extension 32 )

What Obi Devices do i need to make a setup like this work? that from the Main Office i can use one of my Nortel Phones lets say at Extension 15 and Call Extension 30, and the phone at the Remote Office will ring ?

and Vice Versa, Remote office 3 calls Extension 18, and at the Main office the phone at Extension 18 rings?

I know the Nortel's use a Digital line that a normal POTS analog phone wont work with,
Is this possible? what would i need?