Terminal adapters(TA), are to ISDN what modems are to regular phone lines.
Most TA's use the standard hayes modem AT command set, and can be used as a drop in replacement for a modem.
A TA will operate basically the same as a modem except connection and throughput speeds will be much faster than your old modem. You will need to configure PPP exactly the same as for a modem setup. Make sure you set your serial speed as high as possible.
The main advantage of using a TA to connect to an Internet Provider is that you can do Dynamic PPP. As IP address space becomes more and more scarce, most providers are not willing to provide you with a static IP anymore. Most standalone routers are not able to accommodate dynamic IP allocation.
TA's completely rely on the PPP daemon that you are running for their features and stability of connection. This allows you to upgrade easily from using a modem to ISDN on a FreeBSD machine, if you already have PPP setup. However, at the same time any problems you experienced with the PPP program and are going to persist.
If you want maximum stability, use the kernel PPP option, not the user-land iijPPP.
The following TA's are know to work with FreeBSD.
Most other TA's will probably work as well, TA vendors try to make sure their product can accept most of the standard modem AT command set.
The real problem with external TA's is like modems you need a good serial card in your computer.
You should read the serial ports section in the handbook for a detailed understanding of serial devices, and the differences between asynchronous and synchronous serial ports.
A TA running off a standard PC serial port (asynchronous) limits you to 115.2Kbs, even though you have a 128Kbs connection. To fully utilize the 128Kbs that ISDN is capable of, you must move the TA to a synchronous serial card.
Do not be fooled into buying an internal TA and thinking you have avoided the synchronous/asynchronous issue. Internal TA's simply have a standard PC serial port chip built into them. All this will do, is save you having to buy another serial cable, and find another empty electrical socket.
A synchronous card with a TA is at least as fast as a standalone router, and with a simple 386 FreeBSD box driving it, probably more flexible.
The choice of sync/TA vs standalone router is largely a religious issue. There has been some discussion of this in the mailing lists. I suggest you search the archives for the complete discussion.