Do I need a paid account to allow invitees to join by phone?

I have a free account and have successfully invited people to join via computer but two people who only have phones don't seem to be getting the invites or my text messages with the codes on them. Do I have to have a paid account?  I got feedback one time that there are too many people trying to invite people who are using their phones so  I need a paid account, but haven't been able to find verification of this. I send text messages to them and they literally disappear.