Comment 8 for bug 1499342

now, look at that:

I've put my sim card back into the Ubuntu and surprise: o2 now shows up in the cellular settings.

a new scan-for-operators gives this now (I've stripped the still forbidden Telekom and vodafone networks):

phablet@ubuntu-phablet:~$ /usr/share/ofono/scripts/scan-for-operators /ril_0
Scanning operators on modem /ril_0...
[ /ril_0/operator/26203 ]
    MobileNetworkCode = 03
    Name = simyo
    MobileCountryCode = 262
    Technologies = gsm umts
    Status = current

[ /ril_0/operator/26207 ]
    MobileNetworkCode = 07
    Name = o2 - de
    MobileCountryCode = 262
    Technologies = gsm umts
    Status = available

So this seems to indicate a first-time registration issue with the Ubuntu phone, doesn't it?

If needed my wife still has a simyo card which has only been in a Ubuntu Phone ever since the roaming is enabled. I can provide test results or debug logs from that card still, given that mine seems to be activated now.