You should be able to lock down the modem speed to 33.6 or less on a 56k modem. I am pretty sure somewhere in www.usr.com will have the AT settings to do this. I am also pretty sure you can coax OpenServer into talking to a 56k modem, although going through the trouble is probably not worth it.
It is either way good advice to use 33.6 modems though, since that is what the support group is most familiar with. ESG should probably start testing 56k modems with GEMS and VTS since 33.6 are going to become less common.