Not suitable for VOIP numbers of other providers

I ordered the adapter, as the Speedport Hybrid does not have an ISDN connection and I mean behind the Speedport Fritz Box would like to use. Since the Speedport the VOIP telephony but not clean passes to my Fritz box (there is no DMZ, you can not open all the VOIP ports, times klingelts times can one not hear the other side) remained the detour via the ISDN port so that I the above can connect Fritz. So far so good. The establishment of the Speedport ISDN adapter runs all over the online customer center of the Telekom and then here lies the crux of the matter. The ISDN adapter works only with the loans granted by the Telekom numbers because this is configured online and does not rely on the numbers in the router! So once you have besides the Telekom number one more example with SIPGATE etc. this number can not be used as MSN in the ISDN adapter. Therefore an absolutely castrated product and it still totally overpriced.
So if you have a similar situation can help only an analog to ISDN converter which converts the two AB Ports of Telekom router in an ISDN signal which can then be given to the Fritz or other telecommunications equipment.