All DECT handsets available share a common profile - the Generic Access Profile. So you should be able to use any DECT handset paired with your base station to make or receive calls. Other profiles however, are optional and mostly proprietary, so things like caller ID display, automatic date/time settings, the ability to choose the outbound line / set the outbound caller ID may or may not work.
This has absolutely nothing to do with VoIP BTW, DECT is a separate technology and your base station(s) use VoIP connections to forward the calls upstream (or receive and signal them from there), so the VoIP network boundary is at the base station, not at the handset.
Usually, manufacturers are not doing any compatibility tests with foreign base stations or handsets, so you will have a hard time finding any information on compatibility beyond the GAP. The easiest way would be to either just try, or, if you do not feel like trying, just add another base station from a vendor of your liking (Siemens for example is selling 1-line versions of DECT base stations with their IP phone line and has a 4-line version sold separately as Gigaset N510 pro).