Won't they be deleted automatically? I think my Swiss IMSI had disappeared before I renewed it last week. Also I didn't pay attention whether the current IMSI is actually a new one or if I was re-assigned the previous expired one.

In any case I guess deleting an IMSI manually is not that easy as you will need to hack the STK application on the SIM card, which probably cannot be downloaded from the SIM card so it can't be reverse-engineered making such undertaking even more complicated. Perhaps you would need to inject a faked OTA update to delete an IMSI which is also quite challenging, as I expect such to come encrypted.

I am in the US - and the Toggle SIM will only roam on Tmobile, not AT&T. And yes, I have tried selecting AT&T multiple times in a row to overcome steering.

Lycamobile US uses Tmobile only as well, so I suspect they have some special deal with Tmobile. The fact that they have free incoming calls in the US without a US IMSI (I think they are the only mobile operator that does this) suggests a highly negotiated deal with Tmobile.

I have also activated a number of these SIMs in the US (I only kept 2 the rest were for friends.). I have found that it will refuse to register on any network when first taken out of the package and put in the phone. This is because at first it is in UK mode, and it seems to take some time to go into roaming mode. In fact, for the last one I activated I became impatient and went to the STK SIM menu and manually selected roaming. I then changed it back to Automatic later when I had more time.

That's interesting because according to p.2 of this document Dutch Vodafone customers are charged higher rates when roaming on AT&T than on other US networks. Hence you should assume that Vodafone NL's wholesale purchase price for airtime on AT&T is higher and so other roaming networks should be preferred. But there are many possible reasons why toggle does not roam on T-Mobile - one of them could be lacking support of the required CAMEL phase that enables realtime billing, a basic requirement for prepaid tariffs.
Anyway, I expect toggle to start assigning American IMSIs from Lycamobile sooner or later and then customers will be locked to T-Mobile.

I think that Toggle uses Vodafone Global Gateway only for global roaming, not for local numbers & tariffs :S

Please disregard my last post. Obviously I have misread ronwi's post and assumed that his toggle SIM did roam on AT&T but not on T-Mobile. Actually, the opposite is the case. So the higher roaming prices for Dutch Vodafone customers on AT&T absolutely make sense.

So, just to update my post and query. Once I was on tmobile, I was able to use all features. Incoming calls rang (no charges for them) I could enable the data plan I wanted to have already for my upcoming trip this weekend. Easy peasy. Seems to all work pretty darn well. We'll see what the service quality in GB is like soon over the weekend.

thanks for all the help folks.

I heard back from Toggle support - expired IMSIs don't get removed from the STK menu.

It is up to the user to keep track of the active IMSIs through the web site.

This yields the "9 IMSIs" a more interesting question. What if you have 9 IMSIs on the card, of which some are expired, and wish to add a new country as IMSI#10? Works, doesn´t work?

Also, I wonder how the SIM knows which IMSI to use when in auto mode. If it is trial and error, then it might take longer to cycle through.

I do know that using a SIM in the US the first time (as in the very first time) it sits on the UK IMSI for a while before going to the US IMSI.

I don't think IMSI selection is based on trial and error. While or before your phone tries to register on a certain network it must somehow submit the network's MCC/MNC-tuple to the SIM card because all the authentication stuff happens in the SIM card. That's where the STK application running on the SIM card comes in and decides to switch IMSIs if the requested MCC is from a country where another IMSI should be used. Of course the selection will be somehow unpredictable when you are in an border area where there are networks from multiple countries available.