Grandstream are usually well-regarded. I don't have any first-hand experience with their products however. My fear would be if you end up spending a good amount of money and in the end it doesn't really solve your problem.
Check again to see if they can port it. If you log on to your voip.ms account, you can check for portability of a number. Maybe it will be portable now that it's with CC? I'm not sure what their criteria is for determining eligibility.
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