Hello!
You can read out the IPEI with <menu>*#06# on every Gigaset C and S Series. Whith the low-cost Dect - Mobiles this does not works.
I have the same problem. The DECT - phones of Siemens, AEG and all other with DECT / GAP - Standard are working on our external Cordless - System, but i can't use them on the integrated System, because i dont know the IPEI.