There's nothing about this in the General Data Protection Regulation. If you already follow marketing best practices and you've got internal procedures in place to handle and protect personal data as per GDPR, you don't need to get consent from your contacts again.
I've got no specific permission from my customers to send them SMS, push or voice-based messages (my consent is for emails only). Can I message them still?Sure! If your contacts consented to receive messages from you by providing their email address and mobile number, you can message them using whatever channel you fancy. Nothing in GDPR states otherwise.
My contacts are single opt-in, which means I may not have their full, express permission. What should I do?Don't worry! If they've been on-board with you for so long (eg. they're your customers), that's good enough. It's always nice to use a double opt-in form (go for our "GDPR" template!) so you get express permission from new customers, but you don't need to ask old contacts to re-confirm their subscription!
Instead you can simply send a reassuring email to your contacts stating you abide by GDPR fully. We've got a handy template to sort you out! Just create an email off E-goi's templates and choose the one named "GDPR".