Determine who the client is. You will typically have a general guidelines to write to. In some instances, the client will provide an outline detailing everything they would like the answering service to cover for each phone fielded. Your job is to write a script that integrates those elements. It's a good idea to read over a company portfolio before you begin writing.
Focus on writing a script that is easy to read and easy to deliver naturally. The first part of the script will be the greeting, which will typically be simple. A typical opening will be good morning (afternoon or evening), followed by the client's name. Scripts for a business that might receive calls internationally should not include the time of day, which might throw a customer off when calling from a different time zone. Consider using the company name only in the greeting.
Write out several options for the answering service phone operators. The basic options are usually supplied by the client and should anticipate frequent phone call situations an answering service might receive on behalf of the client. Phrase the responses in such a way that the answering service phone operators have room to improvise, but not too much. You don't want your script to back them into a corner, but you want to write specific enough that the purpose of the phone call is satisfied.
Keep sentences in the phone script short. Give the phone operators room to breathe. Long sentences strung together with commas and no periods will have answering service phone operators trying to catch their breath. This will result in sloppy, unprofessional handling of customers. Avoid alliteration and other words and phrases that may cause phone operators to have a hard time repeating your script.