One of our MVPs pointed out today that using Phone.Talk was adding invalid characters at the end of the telephone.
Unfortunately it turns out we had a bug....we were passing a pointer to a string that was not null terminated. This will be fixed, but until then here is the workaround:
To call the number "555-0100" you would need to create the Phone object, and then call Talk as follows:
Phone myPhone = new Microsoft.WindowsMobile.Telephony.Phone();
myPhone.Talk("555-0100\0"); //Notice the null character appended at the end.
-Luis CabreraSDE - Microsoft.
*I did not write this bug...although I have written my share of bugs :-|