Certainly. 2.4GHz cordless phones come in various flavors.
- 2.4 DSSS: Will coexist just fine with 802.11b/g.
- 2.4 FHSS: Will clobber any 802.11b/g on any channel.
- 2.4 FM: Will clobber 802.11b/g but only on some channels.
- 5.6/2.4 cross band: No clue.
From your question, reading between the lines, it appears that you are looking to buy a cordless phone that will interfere. Normally, a microwave oven is sufficient to obliterate communications, but that burns too much power. While I don't condone such practices, methinks the FHSS (frequency hopping spread spectrum) 2.4Ghz cordless phones will do the maximum damage. Most Panasonic phones are FHSS.