Can I continue to use the service in Montreal?

Hi @MLTed

Yes, Ottawa is on Quebec coverage, you don't need a new plan

The only thing to know that you will continue to have your Ottawa number

If for some reason you want to change the number https://fizz.ca/en/faq/how-do-i-get-a-new-phone-number (to have a different prefix, you need to enter the montreal address)

Coverage

Consult the interactive map

• Even if you live within our coverage area, that doesn’t mean you can subscribe to Fizz mobile services. You must live within the subscription area. 
• The coverage area you choose determines the expanse of the calls and texts included in your plan. If you choose the Canada coverage for instance, you can make or receive calls anywhere in the country. But with a Quebec coverage, a call made or receive from Vancouver for example would not be included in your plan, and you’d have to buy a Travel Add-on.
• If you chose unlimited calls when creating your plan, you can make or receive as many calls as you like when you are within your coverage area. 
• The Quebec coverage area also includes the Ottawa region.
• Hawaii and Alaska are included in the Canada + U.S. coverage area, but not Puerto Rico (Puerto Rico is included in the Latin America zone in the Add-ons).
• If you see FIZZ EXT as the network name on your phone, this means you are no longer on the Fizz network. You can use your Fizz mobile services thanks to our agreements with partner networks, but the majority of your usage, however, must be done within your subscription area (whatever your coverage area is). Learn more about the usage limits while on partner networks. 

Source: https://fizz.ca/en/faq/abcs-fizz-mobile-services