Expanding on what leejor said, the 3CX Phone System will check for an SRV entry on the hostname that has been entered and can read up to 3 results returned.
This means that if the hostname you have set in the Management console is sip.xxxxxxx.net, the first thing that it will do is check for and SRV at _sip._udp.sip.xxxxxxx.net. If it does find the SRV, it will follow the priority as set in the SRV. If not, then it will try to resolve sip.xxxxxxx.net to a single IP.
Note: If an Outbound Proxy hostname has been set however, the Outbound Proxy is only resolved to a A Record. Also, in order for the hostname to be resolved to a SRV as described above, the Outbound Proxy hostname must remain empty.
I would ask the Provider is they have an SRV record that included all their servers. If not, I am not sure if creating an SRV an a Domain of your own would have the same result.