I wouldn't worry about it at all. While DECT is crackable, it is beyond the means of all but the most dedicated individual. The risk of your bank or mortgage company leaking the data is higher, in my opinion.

You didn't ask about the security of the VoIP call... But that is encrypted, and unless you are using an unprotected wireless network, the chance of that being intercepted and cracked is too low to be concerned about.