Is it legal for previous provider to ask for cancellation fees if there was no contract ?

It's illegal to ask for penalty for any no-contract phone or internet plans, as well ask 30 days notice.
Prohibition of 30-day cancellation policies
The Commission determines that 30-day cancellation policies for local voice services,
Internet services, and broadcasting distribution services (e.g. cable and satellite
television services) will be prohibited as of 23 January 2015.
This prohibition will make it easier for consumers to switch service providers, especially
for subscribers to bundles of telecommunications and broadcasting distribution services.
It will also contribute to a more dynamic marketplace and remove unnecessary barriers
to consumer choice. Subscribers will avoid double-billing when they cancel their service
and move to a different service provider.
This approach is consistent with the Commission’s prohibition of 30-day cancellation
policies for wireless services set out in the Wireless Code.
Read here:
https://www.cbc.ca/news/business/crtc-bans-30-day-notice-for-cancelling-tv-phone-internet-1.2826054
and the CRTC (law)
https://crtc.gc.ca/eng/archive/2014/2014-576.pdf

Only if you have a balance on your phone, you will have to pay that balance once your number is transferred.

If there was no contract there it shouldn't be any cancel fee.
Maybe you still have a balance on your phone .