Can I configure my soft phone to work with the Twilio SIP endpoint?
Unfortunately not at this point. In order to send traffic to a SIP device, the device must have a valid SIP address provided by a SIP registrar. Twilio is a not a SIP registrar and does not provide SIP registration, so you cannot use the sip.twilio.com addresses generated in your account on a SIP device/VoIP phone.
Instead, you would need to use a different SIP provider that provides SIP registration. From this provider you can make calls to Twilio SIP addresses or receive calls from Twilio to your SIP address.