I know the 7000-series Cisco phones have legacy support. But does anybody here know how to connect an 8800-series Cisco phone (CP-8861-K9) to 3CX?
I can't even get on the webinterface of the phone. Is there a trick to be applied?
These devices need a configuration template, but one does not exist because we do not support those models. Your only option would be to configure the SIP account manually, but if you cannot reach the interface then you will not be able to configure them at all..
The best advice I can offer is to replace those phones with supported devices that can auto-provision, and support all the features 3CX has to offer without any limitations. https://www.3cx.com/sip-phones/
There are going to be more people attempting to hack an "odd-ball" phone, to work with Asterisk (SIP), than with 3CX. It will usually get you moving in the right direction.
If this is a one-off, and you want to spend some time learning the ins and outs of the set, then fine, have at it. If you hope to use a number of these sets, on a working business system, to save some money, then I would suggest you go buy some supported sets, and save yourself some time and frustration.