It's working smooth in my system now.
Since it's a hard modem, no drivers are needed, but demands some manual configuration throught setserial in kernel 2.6.11.5 (I wonder if it would be needed in 2.4 too... in windows XP I couldn't make it work yet, but WHO CARES????)

The USR website has an rpm file for this and other modems. I used that to see if it would go and had no problems. I may mess with the init string to see if I can tweak some better speed. It's on an ECS K7S5A 3.1 MB overclocked AMD but is very stable. Firestarter (firewall) has had some issues with the modem in that it doesn't autorun when I connect, so I have to manually start that to monitor. I usu connect at 49.3 and 52.0, good for dialup. The main thing I can find with the older 5610 (without any suffixes like A or B) is that you can't run the update on the chip to bump to V.92. The best you'll get with this one is V.90. I've had it so long and through so many agonizing config quagmires with 3.1 and 95 that it's seemed a miracle when I found the rpm at USR. Check it out; I hope it works for you.