Persist Access Token Life Time

"can i get access token from back end without opening RingCentral Popup or login page"

Please read http://ringcentral-api-docs.readthedocs.io/en/latest/oauth/#authorization-flows . If you use authorization code flow, RingCentral Popup or login page is inevitable. If you use password flow, you don't need RingCentral login page because you can login via code programmatically. However, not every app supports password flow for security reasons.

Since the Access token has expiration time of 60 minutes , one of the easy and better way would be use Cache and storing it into Cache for 30 mins rather than persisting it into a database.

This will also help in efficient call of API reducing the rate limit issue. Storing in DB will not be a good idea as it will be expiring after 60 mins and so no use of persisting it .

You can also generate the access token from refresh token whose expiration time is longer compared to access token

"other thing is can i get access token from back end without opening RingCentral Popup or login page ? "

For grant type =Authorization code, you will be compulsorily getting login page , but you can you can avoid it if you use grant type =Password