Raiders armed with hammers and crow bars assaulted staff and locked them in a cage for three hours during a £300,000 robbery in Stoke-on-Trent.
One employee was injured and had to be treated in hospital after the incident at the Mastercare building.
Police said up to five men, wearing ski masks, forced the two workers back into the premises on Newstead Industrial Estate as they left on Friday evening.
They stole 2,000 mobile phones and the two staff had to be released by police.
Appeal launched
The two workers, who are in their 30s and 40s, have given statements to police.
The injured man was treated for minor injuries.
Det Sgt Steve Parker said: "This is an extremely serious crime where the victims have been threatened with extreme violence."
He said the gang all had West Midlands accents and left at around 2115 BST in a dark Audi car and a white box van.
An appeal for witnesses has been launched.