To follow up on this story:

Piranha support got back in touch a couple of days ago with instructions to retry sending an SMS message. The phone acquired the SMS Center Number (+48790993080) and I was able to send (and receive) SMS messages. This is the same SMSC # that would not "stick" previously and that kept being replaced as the phone refreshed it's connection with the carrier. I retested this morning and SMS texting worked again (testing was limited to US-based carrier numbers and was using AT&T since it's the strongest signal where I live). Hopefully, others who have had texting issues will find they are also resolved!

Thanks to Piranha-Mobile support for sorting out the problem!

[As a new member on prepaidgsm forum, I ended up writing on the same issue in a second Piranha thread after it appeared that my first post had well and truly fallen through the cracks and was never going to get the moderator approval I needed as a new posting member. I'm posting the resolution of the issue in both threads also for the sake of completeness and future keyword searchers!]

I replied to your PM just over a week ago, apologising for not seeing it or the posts for a few days. During this time I was driving across Europe a couple of times.

But as I said, maybe someone else had approved the posts by then.

The automatic pre-moderation of some posts by new members is to reduce spam, but yes it sometimes delays the innocent from appearing. About two dozen spam ones were removed the other day, and few more since, so there are currently no unapproved posts. There are regular checks, but not necessarily on arriving at a hotel after midnight.

On reflection I've merged the short thread about text issues into here, and removed duplicate posts.

Thanks for merging the posts and for your work moderating here - the general Piranha Mobile Sim Card thread is the best place for it.

I appreciate the need and there was no criticism of the moderation intended in my comment - I was just explaining why I posted twice on the same topic.

But, I didn't post the second time until after emails to the moderator, the webmaster, and info@prepaidgsm had all gone unanswered. Like I said, I thought my first posting had fallen through the cracks and then shortly after I did a second post, both were approved!

Edit: Are the posts by the other posters in the short Piranha texting issues thread still available in the original thread? I can't seem to find it and I'm wondering if they have been deleted or if it's just something temporary while the search engines deal with the recent editing changes.

That thread is fully merged into this one (in chronological order), without a forwarding link from the old.

Posts 325 to 327. Your later ones on that were effectively duplicates once merged here, so just single copies now.

Good idea merging the two threads completely. With the low posting volume, it makes sense to have all the Piranha information together. Thanks.

I've received my Piranha sim today. I did not have any difficulties with the voip setup and the via wifi reception. Then when I decided to test the gsm with wifi off) even if the sim showed full signal, I was unreachable! I fiddled a bit with the settings in the sim toolkit but then I don't know if this solved or not. The customer service told me not to, since the sim should work in automatic. As for the settings on the website, I set:
current roaming profile=Piranha mobile profile 1 (what's this for?)
current translation profile=international
As for the balance, is there a system to check it avoiding to log into the website everytime? Is there something like a short command?

Finally, is the UK number able to receive text messages?
Thanks a lot in advance.

You were already on network before you decided to mess around with the settings, which kicked you off the correct IMSI, all short codes are listed in the F&Qs on the main website. You will find the profile instructions under how to use the rates calculator.

I had to "mess around" because the phone could not lock onto a carrier. You were very quick to reply and I appreciated that but I could not find what you said above into the FAQS.
In order:
how to check the balance from the phone
profile instructions (explained in details).
Any help? As a new customer I want to learn as much as possible about this sim card.

All the information is in the F&Qs please take time to read them fully, you will find how to check your balance in F&Qs under Balance and Numbers. If you look at the rate calculator you will see a button which says "Using The Rates Calculator, click on it, all is explained about profiles.

Sir, it's very easy and I am a customer: is there a short way to check balance instead of using the web site?
More, I asked more on the ticket.
About profiles, it is just said to move between one and another and this is not an explanation for me.
So please, if there's someone else who wants to help, he is welcome.