Especially concerning a child under 1 year. The parents are in different states, but the divorce is being filed by the lawyers in Texas.
Mr. Cord is correct if the case and custody determination is being made in Arizona. Texas has different laws and may have different definitions for "joint custody" and "sole custody". Also keep in mind whether you are speaking about physical custody or legal custody. While Arizona doesn't have a "joint physical custody" determination, I have seen it in other states as a way of indicating equal parenting time and sole physical custody would mean that the sole custodian is the parent that has the kids more often. Legal custody, however, likely deals with decision making like it does in Arizona. You should contact a Texas attorney to clarify with them.