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<begin sales pitch>
Because you can have 
ISDN *and* 
V.34 *and* 
fax (both G3 and the analog stuff) *and* 
cellular communications *and*
voice *and*
touchtone support *and*
caller ID decoding *and*
standalone fax receival with optional printing w/optional 8 mb ram*and*
have optional analog interface *and*
ISDN telephony with the right software *and
a 5 year warranty *and*
flash ROM upgrades *and*
both serial ports
isdn upgradeable
no need for U interface (NT1) *and* more that I can't quote off my head.

<end sales pitch>