I respectfully disagree with the prior poster. Since moving out of the US, I've used the UM mobile extensively and have had a good experience. For many months (until I found a very cheap route to UAE mobiles), I carried the phone with me as a second phone whenever I was expecting a business call from the States. The unit worked very well. The one time that I called customer support, I got through directly.

For many people the problem is that they are in high congestion areas of a city at the worst time. Even if you have a local SIM, if you are standing outside TST at rush hour in Hong Kong, your call may not go through. It is also rumored that some networks prioritize roamers.

A second problem with the UM mobile is that there are very few cheap routes to the card and carriers that provide reasonable termination rates to them often only have one route. When I call a UK landline on most services, they will have multiple routes to a carrier with some fallback. If you are terminating to a Liechtenstein SIM at below $0.20 a minute, you have one or two possible carriers that are options. That is it.

Telestial does a good job for what it is. Remember that most of these internet shops are one or two people shops. They are not massive organizations and for these reasons, customer service can be inconsistent, but when I called them several years ago, they took my call. For reasons stated, I obviously don't buy from them all that often, but when I have, they have worked. I bought one (as a present) for my neighbor's kid when he went to do relief work in Pakistan and it arrived within two days.