Besides some services don't allow you to switch your calling plan easily. There are so many services out there and each one of them has pros and cons. You need to basically figure out what you want in a service and then find the service that can meet most or all of those needs. Don't be surprised if you find that 2 maybe even 3 services that suit you. It'll take time, but if you find someone who is willing to walk you through and explain everything, you will be much happier in the end.
Some customer benefit from using multiple services.
Most of the services out there today do require some sort of contract. Most allow either a one or two year contract for phone service, and anywhere from 3 months or more on a pager contract. On top of that, most contracts have an early cancellation penalty. This penalty can range from a flat fee, regardless of the time left or used on the contract, to a fee based on what is left of the contract. You'd be surprised how many people don't really know who provides the service, or if they are agents of a service or a reseller of the service.