said by DoctorStinky:
Your observation was the subject of some discussion on the old "crippled" thread. And yes, I think the base should determine what features are available through the handset, but that doesn't seem to be the case in Gigaset's world.
Both the included and additional handsets are identified as C610Hs, although the one that came with the base has an appended designation of "CT/CN" while the additional handset's is CT/C4.
There's also a long alphanumeric string below the model number that differs between the handsets:
The base/handset and additional handset were purchased at the same time through Amazon's US site.
I also purchased from Amazon about a year ago. I just tried the code and it worked for my phone.
It appears there is a later firmware.
My additional phone model has the number S30852-S
If you look at your numbers again I suspect the 5 in the second string maybe a S.
The last number on mine is 3 and yours is 7 suggesting a firmware revision.
I am in the process of ordeing a sixth handset frrom Amazon, so I will keep your info in mind.