Can a dual citizen disregard border controls?

You still need to enter the country according to its rules; being a citizen of the country does not waive that requirement. When I bicycle or drive from Canada back into the USA I am required to show my passport to the CBP officers stationed at the border, even though I am a citizen. Likewise if I do the same at an unmanned border crossing, I have to report in as dictated by the signs at that crossing.

As for calling in our your "sat phone", that would not be the same as calling from the "courtesy phone". The officer answering the courtesy phone call would know it is from that particular phone and therefore would know that you are physically in the country. A call from any other phone would not be verifiable proof that you have entered the country.

You asked about the US/Canada and the generic case. There is no generic answer.

• Some countries insist that any border crossing, by citizens or non-citizens, happens at an official crossing point.
• Others agree to special regulations for residents of the border region.

In the United States, 19 USC 1459, 19 USC 1436, and 19 USC 1433 require that anybody entering the US report to a customs officer for inspection. Failure to do so can be punished criminally or as a civil penalty. As a citizen, you have a right to enter the country, but you still can be fined or jailed for failure to follow customs procedures.

The law requires that "individuals arriving in the United States other than by vessel, vehicle, or aircraft shall— enter the United States only at a border crossing point designated by the Secretary." In other words, you can't walk or swim across the border except at a designated crossing, absent special permission. If you're in a vehicle, §1433 requires you arrive "only at border crossing points designated by the Secretary" and present yourself for inspection.

To facilitate this, Customs and Border Protection has special reporting programs for pleasure boats and private aircraft, with various requirements depending on who you are and where you're entering, which can allow you to make the required report by telephone in many cases, once you've registered and received a permit. They've installed videophones at some marinas in the Great Lakes region to facilitate this. A similar procedure is used at the Northwest Angle in Minnesota, which is otherwise cut-off from the United States.

US immigration laws concerning entry without inspection concern only aliens, so they do not apply in the case you contemplate. There are, however, laws that penalize entry without customs inspection. (See US laws requiring US citizens to enter the country at designated border crossings at Law Stack Exchange.)

These are 19 USC 1459 for people and 19 USC 1433 for vehicles.

I don't know about the Canadian side.