Regardless of whether your IT support services are provided by an external organization or by your own internal IT department, this must be covered by an IT Service Level Agreement (SLA).
This agreement can be broadly divided into two categories:
1. Those who define the list of services to be provided with the best efforts
2. Those who claim the same service will be provided, but also support this with a detailed description of the IT service provider's performance when asked for assistance. This is an SLA Backed Services.
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It's a good idea to have a list of things the IT support staff will do for you as routine support and workarounds. However, if it doesn't state when this service will be provided, it may be less than fully useful as a business tool.
When you have a review of your IT service, it can be very difficult to criticize its performance if there is no generally accepted measure of what it should be real. The best efforts for an IT support contract can be avoided even if it looks financially attractive in terms of cost.
To protect your business interests, you must require that every IT support agreement is supported by a contract and governed by a full-service level agreement.
If this support is primarily call logging, progress administration, and management, then you need to make sure that the SLA has obligations elsewhere regarding what is called second-level support.