I had the same experience (Voipcheap to UM+), but was not charged for the call (tried twice). I was able to retrieve my balance.
I tried the same route (Voipcheap) to call UM CS, and they told me that they are having a problem with the system and that engeneers are working on it.
CS said that it would probably take until the end of the week, that's tomorrow :-), so I guess it will be something for monday or so...

Anyway. Voipcheap to UM+ is only 8ct/min (60/60), not bad.

United Mobile no longer selling cards with +423 prefix
 

Hi,

If you try to purchase a new SIM card from UM on their website, they don't let you choose if you want the old card or the new card. I think this isn't fair. Some people may want the old card because you get better rates in certain countries using the +423 card.

Also, did anyone else get a free UM+ calling card for being one of their "best clients"? It came with 10 Euro calling credits, which is a nice gift, but the letter makes it sound like you have to give up your old card to take advantage of this offer. I emailed customer service asking if I could keep both cards but so far I haven't received an answer.

As it looks to me the want to say goodbey to the +423 card otherwise they would not have started the the trading of old cards to new ones.

Fair or not if they decide not to offer any +423 cards anymore it's their choice and there must be a reason for that.
As many people have written they do not tranfer the complete credit on the +423 card to the +44 card. So whenever there is still credit on then +423 card I asume they will not switch it off.
someone from CS did tell me that switchoff of cards is done by hand and not by an automated system.

Not surprised by that at all. They must have just taken it off. As of late last week, you could still get the +423 card on their website. If you really want it though, you should be able to get it via 3rd party.

Even though they allow one to still have the +423 card, I have the feeling they will shut them off in the not too distant future.

I think they (UM) misjudged what customers would do.
I indeed also did not transfer the complete callcreedit-amount to UM+, so giving away UM+ for free will only cost them money.
This way they may be only able to reduce the active UM cards to probably only 50% or so. Maybe they have a problem with the number of users on their platform and reducing with 50% might be the problem? Just a guess... We'll never know.

I think that if they wanted to completely abandon the old card they would force someone to transfer all the balance and not only part of it.

Well I think they discovered, thought or whatever that fewer and fewer people were using the +423 cards due to the unbelievably high price that telcoms instituted to call Liechtenstein mobiles...the question still remains why these prices went up so high so quickly and who is ultimately responsible.

As I have noted, it is interesting that on those services which cannot distinguish one +44 card from another, prices to UK mobiles in some cases remain much higher than say prices to German or French mobiles (see cbw and enlinea rates to UK mobiles)...also my cheap ld service in the USA (1016868 does not seem able to complete calls to either Jersey or Isle of Man mobiles (get a fast vusy signal!)....and then there is the problem still not apparently resolved that when the phone is off, the caller gets a message, at least here in the USA when indeed you are using a service that can complete a call to the Jersey number, "your international call cannot be completed as dialed. Please check the number and dial again or ask your operator for assistance."

What givess? : (did notify UM customer service of this but have not received a response)...

I also have the same problem from here when the phone is off. I think they will fix it. They are just at the first stages of the new card. Please let us know what the reply to your question Matha

I had again the audio issue as I had reported a few days ago. I was at the moment roaming on O2 Czech. Somebody from Switzerland called me and could hear only "Bzzz" sound. Luckly I had his caller ID stored in my phone. Dont know what this can be?

Doh!
 

I just upgraded to UM+, and couldn't find the activation link...so I called CS. The CS agent told me that they were recommending that customers not transfer their whole balance yet, because there are "problems" with the UM+ cards that won't be fixed until "next week". Its a good thing he told me this, because it turns out that the UM+ card doesn't roam in the Dominican Republic -- my main reason for having this card!