The phone has a flat rate subscription, with also lots of free phone numbers, and free access to a number of public Wi-Fi zones. Cost is about $50/month, but it's a corporate subscription with a long list of features.

The tablet has a prepaid subscription with 500MB/month. Cost is about $9/month. I can normally be within private Wifi zones with it. Or I could let the phone supply the bandwidth, making use of the flat rate subscription.

Bandwidth has very seldom been an issue. There are a few times when I drop down to GPRS/Edge. Most of the time, I have access to 3G or Turbo-3G nets.

Depending on client used, I can not only manage VoIP but also video chats while I have 3G coverage.

With a modem in a card slot, and suitable software any laptop would be exactly what you ask. A huge mobile phone with mega-size display. And depending on modem/software, you could either go the GSM route, with standard voice calls with minute costs. Or VoIP calls charged based on your data plan and optionally charged for jumping into the analogue network at an access point potentially in another country, when calling people that do not have VoIP.

Our devices are working quite fine in most parts of Europe, and my mobile equipment have worked quite fine when I have been in US. Most probably, you are either using the wrong operator, or are living in one of the white spots on the map where no operator have managed to set up a decent cellular network.