I had a requirement to assign a SIP address to each AD account on my domain, this is to facilitate a rollout of Skype for Business. The lack of a SIP address stops Skpe (Lync) from autodiscovering the users address. The SIP value is stored in the ProxyAddresses attribute, however you can store it in the msRTCSIP-PriamryUserAddress attribute as long as you have extended your AD Schema.
Without the SIP present, users will see the following when launching SFB (Skype for Business)
After running the script, your attribute should look something like this: