Design Stage

if all you need to do is distribute traffic, you can still achieve this with GTM if your IP allocation is (or can be) tied to each of the lines. Assuming client line has 10.10.10/24 addresses, you could use a GTM iRule to respond to (another assumption ... client has unique ldns servers) client separately:

when DNS_REQUEST {
  if { [IP::addr [IP::client_addr]]/24 eq "10.10.10.0"] } {
     set to client specific public IP access
    pool { client_line_servers }
  } else { pool leased_line_servers }

Been awhile since I've done anything with GTM iRules, so this may/may not work, but hopefully gives you some ideas on how to proceed.