International calls add by month

For some countries you have an option to buy a 5$/month for a N minutes to a specific country.
You can do so by adjusting your plan.
However several users have reported issues with this feature.

You can also buy add on that is valid for 2 cycles but you have to manually buy it. This option seems to be more reliable but more expensive.

The country is listed however I need to know if I can add by month since I do not need all the time

If you go to "manage my account" and adjust plan, it will be on a montly basis. If you go to add-ons, it's only one time.

If you don't use it often then the suggestion is buy Add on.
When you buy and Add on, the number of minutes are valid until the following cycle.
Example : your cycle starts 20th of each month ; you buy International add on on 21st of April ;
that International add on is available to you until 19th of June.

Yes you can , an add-on is valide for 30 day !

You can modify your plan as per your requirement to add international call, but it will become effective in the following billing cycle

Yes you can