simcard routing (bug?)

i got an interesting bug with a mobile phone from my mum. In fact, it is not a bug from the phone itself. Imagine, you are upgrading your phone and want to stay with your number. but you get a new one as well. so we have the following setup: two phones with different simcards from different suppliers (mobile A, and B). You have 2 different numbers, x and y. Now: my mum wanted, the old number y working with the new phone A as well. And so it does. Mobile A is working fine with both numbers, if you phone them. It's ringing as usual and if you phone out it sends the new number x. But, a sms to the old number doesn't arrive at the new phone. Sending a sms to number x arrives at phone A and to number y at phone B. haeh? now i'm completly seems if you change your card you only get some kind of number routing, if your contract is still running But apparently they messed up. The number y is only partially forwarded to the new phone. Come on...what's happening... hack: thought about putting the old simcard into a phone connecting to my homeserver and forwarding all sms by mail, or by my sms-flat... it's frustrating...really frustrating. i made a short video to explain all this and show you: kinda clouded: so long