If this isn't the type of modem you have this might be an alternative that you can purchase. But I would make sure the one you have is configured with the steps below, just to rule out a configuration issue and see if the connectivity issues disappear. Good Luck!
MultiTech MT5600ZDXV modem
Perform the following steps in SAM:
1. Open SAM
a. Select:
Peripheral devices --> Terminals and modems
b. Go to "actions" and select "add modems". Select
the hardware path 20/2.
c. Select the port number 0 and enter the baudrate (9600).
d. Uncheck the "use device for calling out". (In this
case, only the call-in is desired.)
e. Do not select CCITT.
2. No hardware device created (i.e no mksf or mknod used.)
3. No manual inittab modifications.
4. Dial in to check; the HPUX login may appear then hang.
5. Use the following strings to set the modem:
ATZ
AT&F
ATX3
AT&K4
AT&D3
AT$SB9600
AT&W0 (write this to modem's NVRAM)
6. Dial in to check. The modem should operate as expected.