You can move your number if:

• It's an active mobile or landline number
• Public Mobile has service in that area
• It's from a service provider in the same local calling area
• Number Portability is available in that area 

Note:

1. Keep the number live. Don't cancel your current service before switching to Public Mobile. Your phone number must be active in order to move it
2. Have your phone, an invoice from your current service provider and if possible, your service agreement handy when you move your number. Just to be safe, you should review your existing service agreement to find out what charges might apply.
3. Only the authorized account holder can transfer a number.
4. Have your Public Mobile SIM card ready.
5. Make sure your phone works on the Public Mobile network. Please note some iPhone applications such as FaceTime and iMessage may require a plan with text messaging in order to work.
6. Make sure your phone is not locked to any carrier.
7. Have your credit card, VISA debit card or Payment Voucher ready.
8. Check out Public Mobile plans and decide on the plan and add-ons you'd like to buy.
   

To move a cellphone number to Public Mobile provide either one of the three pieces of account information from your old service provider – Old Account #, ESN / IMEI / MEID, PIN along with your alternate phone number and your name.

If you have everything ready, go to https://activate.publicmobile.ca

1. Enter the details required. You will also need to setup a Self Serve Account, which requires an email, username, password, security question and answer, language preference, and a 4 digit PIN.
2. The next step will ask you to choose a phone number. Since you are porting your number, choose the second option to port (or transfer). Enter your existing cellphone number, then click "Check Eligibility". Note that this may take a while. Next will be to enter other details required: Old account #, ESN/IMEI/MEID, PIN, Alternate phone, and Authorization name.
3. After successfully doing the instructions above, you will now select a Public Mobile Plan and add-ons.
4. Review your order, double check everything.
5.  This is the part where you will pay with your credit card. Theres a reward for setting up an Auto Pay. (click here to check the rewards)
6. This part will be the summary of the transaction which also confirms if your account is activated.

sources: 1, 2, 3