Modem is not replying incoming calls

[akmal nerist ernet in]

Hello All, 
I have got a modem and is used to provide dialin connection for
different labs.  It is a old Multitech 224E (2400bps) modem and running
without any problem. To improve the speed, I tried the same with
GVC-33.6KBps and Multitech MT1932ZDX(19.2Kbps) but they are not responding
to incoming calls. Simply they keep on ringing and no carrier is supplied.
But if i use them for dialling out, they are working fine. 
I am using Rh6.0 . 
Someone please help me to use these modems. 
Thanks in advance,
With regards, 
Ashis Kr. Mal. 
Internet Section, 
NE Inst. of Sc. & Technology, 
India-791109