- It's fairly common practice in countries with poorly enforced telco law for people to resell access to the PSTN. They will connect to the PSTN (either via a GSM gateway or a server with an E1 card) and resell that access to a 'wholesale VoIP business'. - That 'wholesale VoIP business' will have an infrastructure, cobbled together with open source technologies with no ability to scale, in the operational or technical sense. - The same 'wholesale VoIP business' will undercut the cost to access the countries PSTN (compared to accessing it directly via a tier 1 carrier) and sell the access to other tier 1 carriers. - The other tier 1 carriers lap up this cheap access, an embrace it because international calls fulfil a small portion of their multi-million revenues. - Therefore, when someone in another country tries to call that country with poorly enforced telco law, it will route via the resold route until it's shut down by the law or it breaks because it's poor infrastructure. - This is currently happening on a big scale in lots of African countries, some central american countries and in the middle east.
I'll leave the context of 'office to office' comms to the other answers as it's been covered in detail elsewhere. But I will say VoIP, all it's forms, is an old technologies, and problems relating to poor bandwidth.. voice quality.. backwards compatibility have been solved for many years.