In such cases, the biggest problem is that the external teams will NOT have the same amount of experience with the product as the product team has, and in many cases, the external team person working on the product has very little or no experience with the product. In such cases, if there is some kind of fast learning program that enables the external team member to learn about the product, it is beneficial, but in some cases, such a help is not possible (primarily because it takes effort to produce such help, and the product may also be changing fast between versions in which this help may need to be updated fairly quickly).
But, consider that whatever be the level of help available, the external team member will take time to come upto some kind of acceptable level, and they will also have a lot of queries on this front. Some queries would be available if there is some kind of FAQ available for the product, but typically FAQ's cover the product as a whole, and may not be available when some external tester starts getting into a feature in detail. In such a case, you will have people who have queries and things will move faster if these queries are settled quickly.
And this is where the frustration comes in. Unless there is some kind of process available for such queries, the team member can feel that these queries come up on a regular basis or get repeated (in fact, when talking to team members, this feeling has been projected again and again and there are repeated request to ensure that these get resolved).
We have tried multiple measures to ensure some kind of process for trapping all these queries, especially with regard to ensuring that the queries are all categorized as per the different features (and this ensures that a database was built up with regard to queries that these external vendors have). First, we tries using a MS-Excel document for capturing these, but then we started running into concurrency problems about exchanging the document back and forth, and who maintains the master copy of the document. And, given that the team was very comfortable with the defect management software and the vendors would also have to start using this software, we laid down a process whereby all the queries were logged as defects against the specific feature where the query was (and with a keyword that identified it as a query from a vendor), these defects were assigned to the respective feature person, and then the query would be responded to from the respective person, and the vendors were also told to ensure that they went through these defects before they raised a query.