>If you're connecting over POTS, you have an analog line; if you're connected to another ISDN connection, then you're using the digital line, which can carry 56K bidirectional (that's the effective data rate of a single B channel.) If you use both B channels of an ISDN link, you can get a data rate of ~112Kbps. But that's not POTS.
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>If you have a connection to an analog line over your ISDN modem, you may simply have a cleaner connection than you have at your office. But even pure digitial, a single B channel is limited to 64Kbps of raw transmission; with framing and other overhead, the effective data rate becomes about 56Kbps, assuming the connection is a pure digital (ISDN device to ISDN device over digital line.) Once you actually have an analog line in the mix, you become limited by the analog line itself; with the curent FCC rules, the raw bandwidth limit is about 52Kbps on the inbound channel and 33.6Kbps on the outbound channel. The numbers in both cases are raw bit rates, of course; with compression, your data rate might appear to be higher. That doesn't affect the raw data rate of the analog line, it's more a matter of how much repetitive data can be squeezed down via compression algorithms (I think that the compression on the 56K standards is still one of the v.42 protocols at best.)

I thought it was just a better connection because it's definitely not twice as good as my connection at the office.