Typically, any high-speed broadband service will be sufficient for a VoIP/UCaaS service. Each call will take approximately 80-100kb of internet. 

You should calculate the number of simultaneous calls you expect to have and multiply that number by 80. For example, if you have 20 phones, you may expect to have a maximum of 10 simultaneous calls at any given time. So multiply 10 x 80kb, and you get 800kb, or 0.8Mb (Megabits). 

The typical cable or fiber optic broadband connection exceeds any speed requirements. A QoS enabled router will help ensure that your telephones are always receiving sufficient bandwidth.