this is the config that works for me (added in dns zone editor of cpanel)
_sip._tcp.voip1.domain.com. 86400 10 100 5060 sip1.domain.com
_sip._udp.voip1.domain.com. 86400 20 100 5060 sip1.domain.com
_sip._tcp.voip1.domain.com. 86400 10 100 5060 sip2.domain.com
_sip._udp.voip1.domain.com. 86400...