Companion phone follow-me

I have a desk SIP phone and a cordless SIP phone, along with my cell/mobile. I’d like to set FreePBX to first ring both the desk and cordless SIP phones simultaneously for 20 s, and then ring the cell for 20 s before going to voicemail. Looking through the follow-me settings, it seems like it can ring them each in sequence or all at the same time, but not 2 extensions followed by an external line. Is there an easy way to do this, whether using follow-me or some other feature in FreePBX?

Thanks in advance!