A partner provides their own mail server. Most emails get sent using the server they provide (SMTP), the client that connects could be from many different IPs. Some emails get sent directly from our own mail servers (only voicemail notifications)
You need to allow IP ranges from the associated link as these servers do change over time.
We have a record they can add to their SPF too
If you currently have:
v=spf1 a include:autotask.net include:sendgrid.net include:spf.protection.outlook.com -all
v=spf1 a include:autotask.net include:sendgrid.net include:spf.protection.outlook.com include:spf.sipcentric.com -all