There are a lot of factors that influence the answer to your question. The short answer is probably !
It primarily depends on the old and new systems, and whether you can interconnect them. It is really a question for the systems integrator engineer that you are using, they will know if the old system can be connected to the new, and pass calls between the systems easily. If you are not going for a new number scheme on the VoIP system, this gets a lot more difficult, but is probably still not impossible.
Speak to whoever is doing the migration for you, and they will be much better placed to answer this question.
yes, u can.