If you want to squeeze out every drop of reliability from your communication services, stick to a landline. They are, by a hair, more reliable, as there aren’t many things implemented into a landline service, meaning that there are far fewer ways things can go wrong. You don’t have to worry about the internet, quality of service, packet losses, or other issues that are internet-related either.
However, if a landline does go down, it will more than likely stay that way for a few hours or days. Adding a new line and new numbers will also require professionals familiar with the tech to carry out the job. Because of this, scaling a landline service is a lot of headaches and hard work, not to mention that maintenance cost is phenomenal. International calls also get a lot more expensive.