What sort of budget do you have to play with?
Some of the best headsets are made by Plantronics for call centre type use.
At 75 to 200 for the headset, the types of phone that will take them can also be expensive.
Similarly, some of the specialist phones-mainly suppliers will only stock the top cost office phones.
However, I think you could do worse than contacting a Worthing firm called Communications Wholesale (Sussex) Ltd on 01903 231587.
They sell items on Ebay (rating 99.9%) and have a knowledgeable staff so if you ask about having a socket for a headset they will know what you mean and know which items have them. They buy all sorts of kit, refurbished, and their website www.commswholesale.co.uk shows a headset plus various phones.
They (for example) would know which cordless phones for home use have SMS capability as well as Caller ID, while few/none of the big firms like Dixons, Argos, Tesco, would know and nor would their staff be likely to find out! The website links to their Ebay items.