There have been cases when you are stuck with a company with just a good website and no real technology behind it. Hence it is advised to follow a few rules while selecting for particular company for providing you with the services of conference call making.
Firstly you must check of the clientele it has. It is good to also know how old the company is and for how long it has been functioning. The number of departments and specially the customer care department must be enquired for. Also one should check that they have their own conference bridges, it is not a good source if the bridges are not their own. You must check for the quality of their service and the availability of it. If possible try to contact the person who is the representative of the company instead of just going by what they have on the net.
The thing you must avoid in all cases is the free connection providers and that too on net. At the time of a business call it is highly unprofessional to get stuck in some technical problem. And these free call service providers do not employ any customer service departments. Hence any speedy help is also not to be expected. In such cases it is always suggested to go for paid services. They get you revenue which would be easy to pay off for the service expenses.
Then you should also check for their billing procedures and ensure that they are fair enough.
At the end, always take up a free trial session, for after all its business you are going to do with it. Hence you must ensure that you take the best for yourself and your employees.