The article is as follows
The new area code is scheduled to be put into service next September. From that time until the following July, there will be a "permissive dialing period" when callers can use either the old area code or the new area code.
After July, use of the new area code will be mandatory.
The question is "what is significant about July?"
a. Use of the new area code will be required.
b. July is not significant at all.
I'm not a native speaker and I'm so confused with the meaning of "until" and "after". So the new area code will become mandatory in July or August?