All VRS providers in the U.S.—including Convo—are overseen by FCC and therefore must comply by its regulations. 

Under Federal Communications Commission (FCC) policy, video relay service (VRS) can only be eligible for deaf, hard of hearing, deafblind, or speech-disabled people. The last four digits of your social security number are the best way to verify your identity to ensure that you are eligible to receive VRS.

You can see FCC's FAQ on this topic in ASL videos here.