troubleshoot gprs Edge modem receiving sms messages

I have a Sentar ST860 Edge modem which works perfectly 99% of the time but sometimes it gives the following error:- "could not set comm state, there may be one or more invalid communication parameters" When this error occurs I have to disable the modem and enable it again and then restart the sms application. Sometimes I have to do this many times. I think it is some other application which is using the same port. Could you identify what application is using the same port or else write some code that would stop the other application from using this port? The existing sms application is written in Visual Basic