I am visiting the USA and I have the Prepaid Allnet M plan on my iPhone. I am on WLAN calling, but still being charged roaming fees while calling German numbers.
Also am I able to port my number now that my SIM Card is active on Congstar from O2?
I hope I understand you correctly. If you call via WLAN, there are no costs. To avoid costs completely, you should disable roaming on your cell phone.
If you want to port a phone number from 02, you can do this by placing a new order via your customer account, specifying the porting. Porting to an existing card at congstar is not possible.