Android 4.1.2
Haven't seen these posted as possible solutions on any other forums I've examined, but I also was extremely frustrated with this same problem...spent over an hour on it enabling everything trying "Starts With" and "Exact Match" options, manual input of the number, just the area code (e.g. for "Starts With" option), copy and paste the number from my contact list (which by the way shows ONLY the (areacode)+7digit number), or import the number from my contact list (which also brings in the (areacode)+7digit number), and then I tried without the area code, and so on...
Even when the Reject List displayed the contact's NAME that the app had automatically grabbed from my contact list, it STILL would not work. Unfortunately, I assumed that by simply showing those grabbed details it "obviously" was confirming it "knew" which number I wanted to block/reject. After all, why else would it grab and show me those details unless it had matched the input "block this" number with the same number in my contact list?
SOLUTION
"Starts With" and "Exact Match" options
It was ONLY after I went back and added the number 1 in front of ALL the numbers when using the "starts with" and "exact match" options. So, the format required was 1 + area code + 7-digit phone number (in the US). I also point out that when I used the Reject List's built-in option to import the number from my contact list itself, it only grabbed the (areacode)+7digit number, and it was NOT blocked because it didn't have the "1" in front of it. Whereas, when I import the number from the Call Log, it DOES include the leading "1". That is inconsistency in the programming which further complicates the issue. On your phone for example, the program might or might not include the "1" when you import the number from the Call Log.
"Ends With" option
Alternatively, assuming your Reject List manager has the "Ends With" option, you can for example, then simply enter the areacode + 7digit phone number, select the "Ends With" option, and that number will be rejected without having to enter the "1" in front of it...thus allowing easy copy/paste from contact list or call log, etc.