Hello, yesterday I've received my UM+ sim card and I can post some impressions.
First of all, sorry for my English, I have few chances to practice!
I've registered the sim on "my account" on UM website typing the PUK code. The sim works perfectly with incoming and outgoing calls, and displays the available amount of credit during the callback. I've tested it with few phones: everything is all right with Motorola V3 and Samsung Z300; using a Nokia 3310, it is necessary to call using the sim menu, but outgoing calls can be done. Also an old Motorola cd920 works correctly, but doesn't display credit. The only phone that doesn't allow outgoing calls is Nokia 6510: direct calls are impossible, so I try using the sim menu, but it's no use: there is always the same message that sounds like: "Sorry, but this doesn't seem to be a valid number", with all the numbers I try with. Another Nokia 6510 of a friend has the same behaviour (version 4.12, one of the latest). I've been writing to the customer care about this problem.
My +447937... number is invisible to the people I call, but the most annoying trouble is that, if I try to call my UM+ number while the phone is switched off, the automatic voice doesn't say it's unreachable, but it just says the number doesn't exist. This isn't good, since people who call me may think they have a wrong number.
I hope my first impressions can be useful. Bye!