For those that have US issued road side assistance plans, make sure you read and understand them before you go. The general rule is, whoever calls the tow truck service is the one that pays for it. So the person needing the assistance needs to call their provider, such as Good Sam if you have their policy.
As mentioned above there are many miles of little or no cell phone service. You flag someone down and ask them to notify the Canadian police at the next town that you need a tow truck. Having the police call for the tow service is one of the few exceptions the road side policies will cover, for them not being called.
I managed to get my truck camper stuck on a soft shoulder of the highway in BC, outside of a cell area for my phone. (AT&T roaming on Roger's Communication, their roaming partner in Canada and visa versa in the US) So probably a dozen or more Canadians stopped to see if they could help. One man said he would stop at the RCMP office in the next town about 50 miles up the road. He did, the RCMP called for a tow truck, the RCMP office had their patrol officer stop by to check on us. He stayed with us for awhile, called the tow company to get an ETA on them and told us he would check back in a hour or so to see if the problem was solved. The truck arrived, extracated us, we paid him by credit card, got a receipt, made sure it stated they were called out by the RCMP. When I got back home to Florida, I called GS Road Service and they sent me the form to fill out. I sent everything back to them and in a reasonable time I received full reimbursement from Good Sam.
If I had called the tow truck myself, somehow, I would not have been reimbursed for the tow. They have to do it this way, to protect themselves from fraudulent tow bills, being submitted. Hard to believe, I know, to think that this gathering, us RVers, of angels, would do anything wrong. LOL But going through the local police agency for the area, actually protects everyone involved in the road side service needed.