Log or find what and which text a SMS app sent? I did accidentially press YES to "[appname] wants to send a message to [premium-rate-number]. Do you want to allow this? [Coin Icon] This will incur additional fees. [Checkbox] Remember my selection. [Button: Cancel] [Button: Send]"

Now it has sent a unknown message to a premium rate number, subscribing me to a expensive service.
The app did send a "START [servicename]" to this premium rate number.

Now its pulling Money from the bill every week. Uninstalling the app dont help.
To stop, I need to send "STOP [servicename]" to the premium rate number.

The problem? I dont know what the [servicename] is. Sending "STOP" is not enough, then it ask that I need to enter a correct service shortword. My operator wont help either, they say "If its hidden from view, you have to Contact the app maker. We wont reveal what shouldn't be for your Eyes". And app maker says I did agree to a lifetime contract with weekly fee, so no recourse there.

Is it any way to log or detect messages sent from a specific app? I did try to install different "SMS Text Tracker app" primarly used for secret spying, but those do only reveal what I send myself through the standard SMS interface.

In other Words, I want to install some logger app or log outgoing messages (even messaging attempts, since sending a additional "START [servicename]" is going to fail). Then I reinstall the culprit app, and then I can find out what it send as [servicename], so I can send "STOP [servicename]" to the number and stop the automatic billing.

Any ideas how to log "hidden" outgoing SMSes sent by app?

And Before anyone mentions it: Decompiling the app wont help. Its obfuscated AND the app does Contact a https server somewhere to get the "servicename".
First, I'd report that app as fraud to Google: such behavior IMHO contradicts with their rules (you can also give the appmaker the option to either have you removed from that subscription, or you would report – with the latter resulting in his app being removed from Play).

Second, if you don't need premium SMS services in general, check with your provider to have them removed altogether. If you can convince them you cannot unsubscribe due to fraudulent behavior of an app having you subscribed to and not letting you unsubscribe, it's their duty to take care for it (to do so, they don't need to reveal that "servicename" to you, as long as they stop charging you). Otherwise you could report them to the corresponding regulation office.

Having premium services disabled altogether should stop them from charging you. After that, if you don't want to read messages coming from that dubious service, you can simply put their number on your blacklist.