The tracert seems to be a strong indicator.

Code:
C:\Users\jn\Pictures\dwn>tracert sip-central01.voipwelcome.com

Tracing route to sip-central01.voipwelcome.com [174.37.45.134]
over a maximum of 30 hops:

  1    <1 ms    <1 ms    <1 ms  192.168.0.1
  2     *        *        *     Request timed out.
  3     9 ms     9 ms     8 ms  te-8-2-ur02.canton.mi.michigan.comcast.net [68.85.85.89]
  4     9 ms     9 ms     9 ms  te-9-2-ur02.vanburen.mi.michigan.comcast.net [68.87.190.118]
  5    14 ms    13 ms     8 ms  te-0-6-0-4-ar01.taylor.mi.michigan.comcast.net [68.87.190.41]
  6    23 ms    22 ms    33 ms  pos-2-8-0-0-cr01.mclean.va.ibone.comcast.net [68.86.90.105]
  7    42 ms    42 ms    42 ms  pos-1-10-0-0-cr01.atlanta.ga.ibone.comcast.net [68.86.86.126]
  8    67 ms    63 ms    65 ms  pos-1-14-0-0-cr01.dallas.tx.ibone.comcast.net [68.86.85.153]
  9    62 ms    61 ms    62 ms  pos-0-0-0-0-pe01.1950stemmons.tx.ibone.comcast.net [68.86.86.90]
 10    64 ms    63 ms    64 ms  softlayer-cr01.dallas.tx.ibone.comcast.net [75.149.228.34]
 11    63 ms    75 ms    63 ms  po2.dar02.dal01.dallas-datacenter.com [66.228.118.207]
 12    63 ms    64 ms    61 ms  po2.fcr04.dal01.dallas-datacenter.com [66.228.118.218]
 13    64 ms    63 ms    64 ms  sip-central01.voipwelcome.com [174.37.45.134]

Trace complete.
Looking fine at the moment from my direction (Canton, MI)
I don't pass through that router.

I suggest to submit a ticket