External Contact SIP Address O365 PowerShell

Hi,

Is there any way to add a SIP address to an external contact on Skype for Business online/Azure AD?

I can add a contact using PowerShell using New-MailContact and add a SIP:<username>@<domain> as an email address but I can't see how to populate the sipProxyAddress for a contact.

I have looked into Microsoft Graph API and all PowerShell options but just can't seem to find out how to do it.

The purpose is to be able to have a list of S4B contactable contacts that appear when using an SRS system on a different Office 365 tenant than the users.

Thank you for any help offered,

Dan