There are only a few options for cordless VoIP phones - wifi are are ok, but DECT VoIP is better.
If you already have a phone you are happy with, you could buy a telephone adaptor (Called an ATA), which converts a standard phone to work on a VoIP network.
For business we recommend Snom phones, as the quality is hard to beat. They can be a little pricey buy quality is important when speaking to customers.
Siemens also offer great quality cordless phones, such as A580IP DECT Phone. The siemens phones are great as they can connect to a land line as well as VoIP so you don't have to juggle handsets. Its also good for expansion as you can make two VoIP calls at the same time by adding additional handsets. They cost around £54 + VAT + P&P
As we are a VoIP provider I can recommend both products from experience.
I ditched the idea because I couldn't get it to work and simply it was costing me more in time trying to fix it and emailing the provider, than the amount og money I would save. Not worth the hassle IMO until this technology is a lot more advanced than what it is.