Can I keep my cell phone number for Virgin Mobile when I switch over to Telus?

Yes you can but you need your PAC code, ring virgin up and ask but they may charge you if you haven't used the number with them for at least one year. Then, ring up telus when you have your sim and tell them that you want to change your number and provide the PAC code.

Through the network porting process you can keep your phone number after changing your network.
For porting,try to do the following steps.
1.You will need your PAC(Porting authorisation code) from your current network service.
2.Then contact your new mobile service and tell them your PAC.They will provide your new sim card.
After getting your new sim card,Make sure your phone is unlocked one.If it's restricted by single network means,try to unlock your mobile before swap your sim.For unlocking visit UnBBLock.com They will provide an unlocking service for Blackberry mobile models only.If you have other than Blackberry phone use Unlock-Zone.com