An internal hotline extension can only be a 2 digit extension or loudspeaker page number, so you are barred from entering it as data.
To get around the restriction, you have to make it jump through a couple hoops via external hotline.
Take an extra extension port and loop it back to an extra CO line port. On the hotline phone, remove all line appearances except for the looped extension-CO line. Delete any entries for the hotline extension under #603. Assign the hotline extension under #311. Program the personal speed dial 80 for the hotdial extension from centralized telephone programming at station 10 or 11 using Feature 80 Hold 771. The hold will insert a pause (if needed) so dial tone can be established before the 771 is output. Of course, modify the hunt group as needed.
Since system phones use a digital hookswitch code to tell the system a multiline phone has gone off hook, use an analog 2500 set (no dial pad is preferred) for the hotline phone. Otherwise the system will never see the phone in use to command the hotdial to begin.
This will page a series of phones, not multiple phones at once. Substitute a calling group for the hunt group for simultaneous ring of several extensions.