How to use headphone-mic combo on a landline phone?
"Please tell about phones which do not have 2.5 mm connectors" - Sunil
If your handset doesn't have this jack, you will either need to get a handset that does, or add another phone that does. I don't recommend "hacking" a handset to add a 2.5mm jack or splice in the wires from a headset.
Thanks. I understand
Any handsets compatible with headsets should have documentation included in the instruction manuals. There may be headsets specially made for handsets without the 2.5mm connector, but you would have to check out the manufacturer's website and/or the Plantronics website as mentioned by Oron. Otherwise, you would have to purchase a headset-compatible handset.
Some handsets have 2.5mm connectors to allow external headsets with microphone to be used. These phones would have instructions and more information about compatible headsets, how to connect them, and how to use them within the documentation that came with the phone.
Thanks. Please tell about phones which do not have 2.5 mm connectors
You need a special headset (or an adaptor) for this to work. The premier manufacturer for those is Plantronics. Check out their main site, which I'm sure will have an explanation about the details.