You can call into customer service and request a seasonal hold plan for those 6 months for $15/month. Keep in mind that this is a plan change, so you will lose the plan you currently have and will not be able to use it again. You will have to select an in-market plan when you return.
A workaround is to port that # out to prepaid and pay the $15 base plan for the first 90 days, then renew it again just before the next 90 days is up, then add the line back to postpaid when you return. This will still result in the loss of your plan but it's a cheaper way to keep your phone #. However, by porting out and using this method, you'd have to pay off any outstanding Tab balance, so just be wary of that.