International cell phones

Is there a way to add funds to an old sim card from masmovil that we used last year prior to getting to Panama?  We have an iPhone 5S.

Most carriers reserve the right to cancel a SIM card/phone number if it has not been used in the previous 30-60 days. Do not add any money to the SIM card or number until you get to Panama and verify the number is still valid. I know, I lost $5 that way last month in Nicaragua.

Thanks!  Is there anyway to do this here in the states?  the last time we went to the Masmovil store it took over 2 hours.