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Talk about a FUBAR'd situation, I can understand your frustration and would feel the same in your shoes.
The only leverage you may have is to gently remind whoever you speak to, that you can switch to another provider and will unless they help resolve this. Even though you've called already and spent hours via phone/emails, continue that effort, asking to escalate whenever possible. Hopefully you can find someone in their system that might understand that you're willing to pay the collection account but only if it's removed.
I'd scour the web as well looking for Upper Muckity Mucks email addys within Frontier and write them a well thought out email explaining how this all happened and ask for their personal help.
Sucky situation, hope it turns out for the good for you both.