Cruel thought here: Do you use Marketo only for SMS & Co.? Then it's most likely not the right tool.
I agree with Michael, let's start with that question before we start talking about how to lookup on Phone fields.
Initially we want to start with using it for SMS and mobile app, but will move towards email and other channel eventually. We have a very small customer base and not very high throughput requirement.
If Marketo still fits your budget, that makes sense. But not using the platform — at least in part — for email is uncommon!
Frankly the easiest way to get what you need is to store the mobile number in the Email Address field as well as in the standard Mobile Phone. Then also have a field like Email Address Category that holds either “Email” or “SMS” to tell you which type of data you’re currently storing.