Does Microsoft Fax Service on 2003 accept simultaneous incoming faxes from multiple modems?

Hi,
I'm looking into consolidating 2 xp workstations which both have modems receiving faxes and a fax machine into one 2003 server with Fax Services enabled. I'm happy with how it works, having tested this with an external winmodem.
But before going ahead and purchasing a load of external usb controller-based modems, so that I can configure multiple incoming lines on the one server, I need to know if consolidating the fax lines this way does not hurt the volume we can currently handle using effectively three devices.
I know that 2003 server can take 4 lines, or "no limit" if you have 2003 enterprise (as in http://support.microsoft.com/kb/555403), but how about the availability of the fax service when one modem is sending/receiving?
Can 3 external controller-based modems (such as the US Robotics USR5637 dongle) receive 3 faxes simultaneously? If so, is there a bottleneck in the fax service itself (can the service process sending/receiving in multiple threads, or are they all queued up?).
If there is such a bottleneck, then the 3-modem faxserver would not serve our purpose, since we wouldn' t be able to process the same volumes as we do now.