I've helped several businesses do this.
1) Differentiate yourself - make your portal highly compelling for small-to-medium businesses.
2) Give it for free to businesses that are recognizable and will allow you to feature a testimonial from them. This will build credibility. I like to go after a single location in a chain, for example get a single McDonald's manager to use it, a single Chipotle's store, a single Applebee's manager - and now you have a powerful social proof client roster.
3) Do you need a lot of users (people with the phone app or who are using the tech) in order for businesses to benefit? If so, that presents another set of complex challenges. I can help you create a strategy to get to market and revenue fast with this challenge by phone. It's the number 1 reason startups like yours fail - unable to get traction from both users and businesses simultaneously. If you don't need both sides at once, then...
4) Get feedback from the free users, then ask them if they'd be referral sources. Then begin an outreach campaign - you can use techniques from Mixergy or set up a call with me - it will require a custom plan based on the specifics of your business once you reach this point.