This draft is a bit unusual: it reads like an informational document, but there are a few instances of RFC2119 key-words (about https URIs at the Call Placement Service (CPS), and (m)TLS between OOB-AS and CPS), so I guess the PS status is OK even though a bit "weak."

The two requirements around the use of TLS in §5 could do with a reference to 9325.

Nits:

# Section 1

s/transitting/transiting/
s/to the PSTN,/to the PSTN/

# Section 3

s/but nor does/nor does/
s/PSTN/Public Switched Telephone Network (PSTN)/

# Section 4

s/services providers/service providers/
s/numnber/number/
s/originating domains has/originating domain has/
s/within a scope of an SPC/within the scope of an SPC/
s/TN based/TN-based/
s/signed by a CA, and contain their own/signed by a CA and contain their/

# Section 5

s/caller (or callee), to the degree/caller (or callee) to the degree/
s/A CPS can use this mechanism can authorize/A CPS can use this mechanism to authorize/
s/connection, if there is/connection if there is/
s/Furthermore, use of/Furthermore, the use of/
s/although use of transport-layer security/although the use of TLS/

# Section 6

s/means that called parties can acquire PASSporTs/means by which called parties can acquire PASSporTs/
s/or through a subscription interface/or a subscription interface/
s/calls arrive, or receive/calls arrive or receive/

# Section 7

s/legacy providers, and also/legacy providers and also/

# Section 8

s/for contributions/for {his,her,their} contributions/

# Section 10

s/an entity contractual acting/an entity contractually acting/
s/telephone numbers ranges/telephone number ranges/