Office 365/IIS SMTP relay - FROM address 1235551212@contoso.com not allowed to relay

I have an old VOIP system that sends voicemails as email links/attachments ...
the FROM address (prefix) is always thephonenumber@contoso.com
How do I allow IIS's SMTP virtual server to relay (to Office 365) with the FROM address constantly changing?
All devices can relay just fine.  
Only this VOIP system is an issue (where I cannot keep the "FROM email address" constant)

Until now, I always used scenario 2 in this guide to successfully setup SMTP relay with Office 365:
http://o365info.com/smtp-relay-in-office-365-environment/

VOIP:
Toshiba Strata CIX
Stratagy ES Voice Mail Application
http://cdgenp01.csd.toshiba.com/tsd/docum/CIX-Documentation/CIX-MA-PRGM2-VA1.pdf
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K BAsked:
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Vasil Michev (MVP)Commented:
You can configure it as explained in the "SMTP Relay" section here: https://technet.microsoft.com/en-us/library/dn554323.aspx
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K BAuthor Commented:
Vasil, you rock!
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Casey WeaverManaged Services Windows Engineer IIICommented:
Did this solution work for you? We're running into this issue on a Strata, but using a relay doesn't work because O365 will say that the sender is not permitted (because of the ever changing email address matching a phone number in the FROM field. Any way to get around this?
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K BAuthor Commented:
can you send screenshots of your IIS SMTP relay?
Yes it is currently working now
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