Basically yes, but there are exceptions.
Number portability is the ability to still use your phone number from one phone service provider with another. This is fortunately possible today between phone service provider companies, whether they offer wired or wireless service. The regulating body in the US, the FCC, recently ruled that all VoIP service providers should offer phone number portability.
Some VoIP companies do offer number portability, against a fee. The fee charged can be a one-time payment or can be a monthly fee payable as long as you keep the ported number. So, if you care a lot about number portability, talk about it to your provider.
Your records should be the same
One very important thing to know if you want to keep your existing number is that the personal records of the person owning the number should be exactly the same with both companies.
For example, the name and address you submit as the owner of the account should be exactly the same with both companies. A phone number is always attached to a person’s or company’s name and address. If you want the number with the new company to be, say, that of your wife, then it will not be portable. She will have to use the new number obtained from the new company.
Now, read on what could prevent you from using your existing number with VoIP.