whatever your choice i advise you to avoid xxsim, is a waste of money.

Over the years, many LD carriers in the U.S. simply blocked calls to +372 mobile numbers because of complaints from customers about high charges. Don't know if that still happens. What I want when I travel is forwrading my U.S. number to my SIM card (whoever's I'm using, w.o. having connection problems. Obviously I would need to forward to a SIM w. a U.S. number.

piranha mobile would be perfect for you.

Piranha is the only international forwarding card I know of, and I've looked at google voice and other quasi solutions mentioned around the net.
if Piranha is my best option when abroad, I forward directly to it from my US phone, but was recently in the UK and used an Orange UK sim - but still used the Piranha sim to forward to the UK sim. It's worth buying just for this!

I have used Vanity, 1-800 Numbers, & Toll Free Phone Services – Kall8 for a call forward service for years. It will go directly to a foreign phone number. It has a 800 style number that it uses. I have received calls in Duabi, London,
Russia and all of the Old Soviet Block countries as well as Afghanistan. So you really don't need to forward to a U.S. number.

Like Snidely mentioned, most carriers either do not connect to these +372 numbers (and similar) or charge a hefty premium to do so. If I read it correctly, Kall8 charges $0.55/min plus whatever the other charges kall8 charges, to call a +372 mobile.

Betamax / voicetrading is again offering access to +447 numbers. Had problems the last 2 month calling these number. Complained and finally got an email a few days ago saying it's fixed but they are using a higher rate for this number.

voicetrading is now charging around 15 cent per minute.
That said eu rates for incoming calls inside euo is max 9 cent.
Using voicetrading to call me dutch or belgium mobile cost around 3 cents. Total 12 cents.
This means that whenever inside europe it might just be cheaper to use a regular eu sim and any provider can only charge 9 cents for incoming.

That said a uk number might still be cheaper than an estonian. Also giving out a +44 number just looks better.

2 other options?
 

I believe that both WorldSIM Call Forwarding rates
and OneSimCard rates
have the ability to forward calls also.

Their forwarding costs might be prohibitive for the destinations you want or need.
[I recall WorldSIM being free forwarding to the US at one time a few years back. And Canada is $.00 still.]

And both have higher cost of entry--that is--the cost of aquiring the SIM in the first place.
And might have charges to keep them alive that you would rather not pay.
But, at least one can compare costs and realise the value one might be getting.

After reading these Piranha threads I am getting awfully close to ordering one.

I went ahead and bought one today.
 

I went ahead and bought one today.
Price went up nine cents between the first time I looked today and I purchased it. Free shipping is available.

Anyone here can explain balance and SIM expiration?

Many US cellphones can't forward to US 800 numbers. Don't ask me why. You can sometimes get the blocked lifted by level 2 customer service, but I find it is easier to just use a standard phone number. Callcentric is giving away US phone numbers (New York) which can do international call forwarding.