Public Mobile uses the Telus network. The actual hardware, towers and antennae and wires and computing and networking stuff.
Telus has a network sharing agreement with Bell (and SaskTel, etc) which allows them to each provide full "nation-wide" coverage without having to deploy/maintain network hardware on a full "nation-wide" scale.
But they're Canadian companies which only operate in Canada.
There are some Important Big Organizations which define international telecomm/datacomm industry standards at technical levels, but basically if you want to do business in another country then you've got to abide by whatever laws and regulations and conditions and and costs they (their government and their communication industries) will impose.
SMS texting is fairly global because it's derived from GSM standards which were adopted almost everywhere. And SMS packets are discrete with small (fixed maximum) content, they're wrapped in standard digital envelopes, they're easy to count and sort and filter, they're easily transferred between networks. So it's easy for local telcos and foreign telcos to make agreements which allow texts to cross borders.
Talk is something different. Still fairly low bandwidth but not as trivial as text. Also must be realtime and open-ended, a conversation doesn't automatically announce exactly how much time (data, money) it'll consume. So it occupies more active network hardware on both ends (and everywhere in between). And different companies have different costs for such stuff - local or long distance (however they defined or designed their networks) - so different agreements need to be reached with each one.
(Indeed, certain countries actually obstruct attempts to communicate without restriction across their national borders, or they at least resist allowing foreign companies from providing communications they can't directly monitor or control.)
The big USA carriers already have Unlimited North America Talk and Text. It's obvious this will eventually trickle down to the little USA carriers. And it's obvious that if their prices keep dropping then they will reach a point where Canadians find their services cheaper and better than the services being sold by Canadian companies.
I doubt unlimited intercontinental or unlimited global talk will happen soon, but data keeps getting faster and cheaper (while data tracking keeps getting more lucrative) so maybe it's possible within a decade.
