another option...just before you leave you can future date a plan change to $15 plan. Manually pay $90 ($15 X 6) into Available Funds so that the plan will renew as normal for the period of time you're gone. When you get home, you can then select a plan that you like.
It's only $90 and then there's no playing around with remembering dates etc.
@MaureenC You can suspend for 90 days at a time, the account will suspend if you disable autopay if you have a CC on account or simply don't make a payment. However, you need to make a manual payment around day 86 of suspension if you don't want to lose your number and account. This can be repeated for another 90 days (not funding the account) but keep the deactivation date in mind always.