Here is a list of SIP to PSTN termination providers offering free SIP termination to North American toll-free numbers, along with a format example of how to call 8005551212 (a fictitious number) using each.
*18005551212@fwd.pulver.com
18005551212@proxy01.sipphone.com
16416418005551212@sip.tollfreegateway.com
18005551212@tf.voipmich.com
18005551212@tollfree.sip-happens.com
8005551212@voiper.ipkall.com
Here I should mention something about caller-ID, because caller-ID can affect whether the call is completed normally.
As a general rule when a call is attempted, the termination provider will examine your caller-ID number (the "User ID" number of the calling SIP device) to see if it appears to be a valid
NANPA number. If your caller-ID number does not appear to be a NANPA number then the provider will substitute one of their own numbers in place of your caller-ID number. Some SIP numbers that look like NANPA numbers (such as Gizmo5 1747xxxxxxx numbers) may be passed to the receiving end.
Typically, the receiving party's equipment will also examine the caller-ID number and will answer the call ONLY if the number is from an area code from which they wish to pay to receive calls. The result is that when calling some toll-free destinations through some providers, you will hear the phone ring but your call will not be answered. If this happens try placing the call through a different provider, or try changing your outbound caller-ID number to show a different NANPA area code.