Other issues to look out for
Check to make sure that the company you are dealing with is reputable – do they provide contact details?
Where are they based?
How experienced are they?
Find out how much they charge to set up the number, and then what their monthly charges are after that. Shop around – there are a lot of companies out there who all want your trade!
Many companies offer "memorable ghost numbers". Whether or not you need these depend on your individual circumstances – they are best used on TV and radio campaigns. Don't be tricked into believing you need one. Also consider, is the number actually memorable?
Consider the contract – can you request to leave within a month, or do you have a minimum contract of a year or more?
Look at how long the company takes to set up the number – this really should be a relatively quick process.