College Hill partner Adrian Duffield is leading the account and reports to ATC CEO Alex Tupman.
Tupman plans to raise £3m from the share placing, which will be spent on working capital and acquisitions in the fragmented voice over internet protocol (VoIP) industry.
ATC focuses on corporate telephony for small and medium-sized businesses, which typically have 50 or more users.
The company believes its focus on VoIP has given it the edge on more traditional competitors.
ATC has 2,300 clients in the UK and claims a customer churn rate of just 5.35 per cent against a sector average of 22 per cent.
It seems to have benefited from a switch to VoIP among UK businesses.
So far this year, nearly 93 per cent of its new system sales have been IP-enabled, compared to 62 per cent last year.
The group quadrupled its pre-tax profits in 2004, recording £947,000, up from £215,000 in 2003.
Tupman said the listing would provide funds for more organic growth and the takeover of industry rivals, as well as funding an employee share option scheme.
'We have high visibility on future earnings and our current trading is strong,' he said. 'As businesses refresh their technology, we are well positioned to take advantage of the growing demand for IP services.'