My friend and I are from Russia, which is why we can't verify our phone number.
For me, this is just an additional step that I would like to take, but my friend can't play Iron Banner with me because of this problem.
How can this issue be resolved?
Are you getting a error? If so, what is the error? In my experience Bungie's form for this leaves something to be desired and frequently bugs out.
I recommend using Chrome incognito (to prevent plugins from interfering / cookie sessions bugging anything) and entering the phone number, waiting a second, then clicking the submit. If that doesn't work, then try using the enter key (or vice versa).