The support page of the official product lists all the available modems that you can use with the software/server. The list is based on extensive testing among the time. Please check the list and see what is good for you.
As a results, here a few models that have good compatibility with the software/server:
Digi Acceleport Ras 4/8 Port Modem cards
Dialogic Diva Media Boards 2/4/8 Ports
Mainpine IQ Express 2/4/8 Port Modem
Multi-Tech MultiModem ISI Multiport Analog Modem
Comtrol RocketModem IV
Perle PCI Ras 4/8 Port Modem
ISDN: AVM-Fritz PCI or -USB (2 channels)
Zoom 3048/3049/2949/H08-15328/H08-03328
Equinox MultiModem
MainPine RockForce
Intel HCF 537EP V9x DF PCI
Eicon Diva 4BRI
Dialogic® Diva® V-PRI/E1-30
