User blog comment:B214/Volume 22 spoilers/@comment-1884090-20160804145625/@comment-9595671-20160804170224

They also likely needed a way to reach Gabriel, Ajuka and Shemhazai in case they need to reach someone. Like they did with Ajuka so they could give Issei the message they did in this volume.

It's also probably a way to still have some of their influence remain while still trapped and fighting against Trihexa... I hope they can tough it out, even if they don't get out soon they'll hopefully get out alive.