You could do all of the above from your end.
Example
Option 1
Using CNAME on your DNS
Domain: usa2k.com
CNAME: sip -> Address: sip-byod.voipwelcome.com
This allows you to use sip.usa2k.com address and if Tim changes the IP, you don't have to worry since it points to his name record and not IP.
Option 2
using "A" record on your DNS
Domain usa2k.com
A Record: 67.228.77.18
This allows you to use sip.usa2k.com and look like you own the server at a glance. Reverse look up still reveals sip-byod.voipwelcome.com but you cant change that. This is a common method in virtual and hosted server space.
Draw back is, if Tim changes his IP, you need to update that IP on your DNS entry.
sip-byod.voipwelcome.com and sip.voipwelcome.com shows up as an A record however the reverse look up comes back with sip-byod.voipwelcome.com. So for CNAME, i would recommend using sip-byod.voipwelcome.com but for A record, it doesn't matter since the DNS entry is an IP.
sip-byod.voipwelcome.com IN A 67.228.77.18 3600s (1h)
sip.voipwelcome.com IN A 67.228.77.18 3600s (1h)
Hope this helps,
M
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