That board is specifically for E1, not T1, so it won't work in North America. If you are running outside NA, that is probably just what you want. (I just wanted to clarify that point)
At no point should you start experiencing dropped calls just on a call volume basis(the question actually surprises me a little, as we have never had a problem like this). Granted, that could be an oversimplification: If something you are doing in the application causes some form of resource issue, it is theoretically possible to have dropped calls based on call volume. This isn't something I could predict, having not developed your application(s). What I can tell you is that TeleFlow and the hardware itself will not start dropping calls simply on a call volume basis.
I suppose it is also possible that an external factor could come into play with dropped calls. If you had a PBX between the TeleFlow Server system and the PSTN, the PBX might drop calls over some volume. Again, this isn't something I could predict.
The one reason I can predict that you will get dropped calls is application errors, which again probably don't relate to call volume. A thoroughly tested and debugged application will eliminate this issue.
A final point: I believe you had some plans to use TeleFlow for Speech Recognition. Only prompted SR is supported in TeleFlow with Dialogic hardware. For barge-in, you have to go with NMS. Furthermore, you have a little more control with outbound calls in TeleFlow using NMS, if that is a concern for your applications.
Here is a link for a similar card from NMS Communications:
CG 6060