I ported my cell number to Google Voice about 8 years ago when I moved out of the US, and it’s always been detected as a VOIP number and annoyingly blocked by services that block those. I ended up getting a cheap prepaid cell plan with voice/sms over WiFi support the last time I was in the US just to have a number to use for SMS verifications and anything else that blocks VOIP numbers. I wonder if the service you’re using is doing something different than GV or you just got lucky with the VOIP detection.
Edit: Just took a look and their pricing is very reasonable. I’ve been de-Googling my life over the past year specifically out of fear of what happened in TFA as I realized I was relying way too heavily on them and I have zero trust left in Google. I didn’t port my GV number to the cell provider because it works for most things and it’s more convenient. This looks like a solid option and would only cost me a dollar or so a month at the amount I use that number.
Edit: Just took a look and their pricing is very reasonable. I’ve been de-Googling my life over the past year specifically out of fear of what happened in TFA as I realized I was relying way too heavily on them and I have zero trust left in Google. I didn’t port my GV number to the cell provider because it works for most things and it’s more convenient. This looks like a solid option and would only cost me a dollar or so a month at the amount I use that number.