No. All intake workers who are aware of the client's experience with homelessness can offer 3rd party verification of homelessness. There is no rule that the verification has to come from a separate agency.
Third party verification from a homeless service provider is more flexible than other forms of 3rd-party verification. While third parties in the community (ex: friends, neighbors and other people experiencing homelessness) must have physically witnessed the client's living location, homeless service providers may certify that a client is homeless even if they have not been to the client's living location. To do this, they must sign a statement explaining where they did encounter the client (such as in a drop-in center or at the provider's facility) and why, to the best of their knowledge and based on their professional judgment, they believe that the client stayed in an eligible homeless location. However, because the provider is the one asked to sign a statement and affirm the veracity of the client's homelessness, if the provider, for whatever reason, believes that the client is not actually homeless, they are by no means compelled to sign a statement on the client's behalf.
Please also note that third party providers may not sign for any months of homelessness prior to commencing services. For example: if a client enters a drop-in center today, having never engaged with a service provider before, but states that she has been homeless for the past 2 years, the provider may only certify the client's homelessness as of today. However, suppose the client has been visiting this drop-in center for the past 2-years. In that case, the intake worker may certify the client's homelessness even if they have never physically witnessed the client's specific living location in that time.
Verifying homelessness without physically witnessing the client's location should be saved as a last resort but remains an option for clients who are challenging to verify, either because they are particularly transient, hard to locate, or because they are not great historians of their own homeless history due to disability.