Excerpts from netbsd: 17-Apr-98 Internal modems? Paul
Goyette@whooppee.co (1775*)

> 	com0 at isa0 port 0x3f8-0x3ff irq 4: ns16550a, working fifo
> 	com1 at isa0 port 0x2f8-0x2ff irq 3: ns16550a, working fifo
> 	isapnp0 at isa0 port 0x279: read port 0x203
> 	isapnp0: <U.S. Robotics Inc. Sportster 560, USR2070, , > port 
>		0x3e8/8 irq 5 not configured

The modem uses port 0x3e8, so it is probably neither
com0 nor com1. (Unless it is similarily broken as these
3com PnP cards.)
I'd add the "USR2070" to the list of known devices in
dev/isapnp/com_isapnp.c:com_isapnp_match() and see
if it helps.
(It needs a "com* at isapnp?" in the config file.)

best regards
Matthias