I have been in touch with MP support on that problem as they support smspva api and I had the same issue with PV
There are some solution
The IG verify url used to be i.instagram.c0m/challenge/*USERID* somedays ago they changed it to i.instagram.com/challenge/*userID*/*randomstring* - it was working for some time, lets say a day.
Now the problem came back, I had PV loops on some of the accounts yesterday [5 PV, caused by proxy subnets- if You got the loop just change the proxy- thats confirmed] , also got 5 new PV today, they went without problem and i noticed the url change to i.instagram.com/challenge <- only, and it worked without any problem, managed to PV the ones from todays. However.
The accounts from yesterday seems to be somehow still broken, I used to bypass the problem by loging it into the emulator, and doing the verification via instagram apk, but now, even on the apk it prompts with that error- Sorry this page isnt available, which is weird. Also if Your seeing this page, and using MP, try hitting "Go to instagram" in embeded browser, it shall work as it work for me lately and today. Not sure what caused the 5 accounts from yesterday to jump into the shithole of not being able to log and see the phone number form.