Trunks are essentially the same thing as saying telephone lines.
Why are trunks in your call centre important?
A lack of trunks can also result in customers being unable to access your call centre at all.
If there are not trunks left, a customer will receive a busy/engaged signal.
Newer cloud solutions for call centres also need trunks, however, they rely on SIP trunks which are far less costly to purchase and maintain.