I joined Visible in Feb 2021.
I still have the old plan with "Party Pay" that was available at the time and have not upgraded to one of the current plans.
Am I able to call Mexico, or do I need to upgrade to a new plan first?
The old plan included calling to Mexico and Canada so no you should not have to switch to a new plan. If you plan on going to Mexico then I don't think that plan allowed roaming there or Canada and that would need the new plan(the Plus plan I think).
Also, it doesn't sound like they are ending this plan anytime soon. I'm still on it and remaining on it as long as it is available. I posted a video in the "Let's chat" section from the chief commercial officer. If you wish to know more the link is in this thread, https://community.visible.com/t5/Let-s-Chat/A-YouTube-video-about-the-Legacy-Party-Pay-plan/td-p/336...