We had a German customer very recently asking for this feature, so here are some observations:
Twilio
Nexmo (Vonage)
MessageBird
Plivo
Sinch
These were the service we looked into, as they offer some kind of integration with Marketo. Also to consider: Is it only Marketo --> SMS that you want to push or do you also want a back channel for responses or unsubscribes? How do you obtain an opt-in for SMS pushes in GDPR areas? Do you want SMS to be your destination or also Whatsapp?
Not an easy topic AFAIK. And I would always ask: What do you expect to achieve with SMS that cannot achieve with email?